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diff --git a/backend/tests/test_calendar.py b/backend/tests/test_calendar.py
index 35f3bec..d4685a7 100644
--- a/backend/tests/test_calendar.py
+++ b/backend/tests/test_calendar.py
@@ -1,7 +1,8 @@
 from fastapi import status
 from fastapi.testclient import TestClient
 from sqlalchemy.orm import Session
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone  # Add timezone
+from pytest_mock import MockerFixture  # Import MockerFixture
 
 from tests.helpers import generators
 from modules.calendar.models import CalendarEvent  # Assuming model exists
@@ -10,13 +11,14 @@ from tests.conftest import fake
 
 # Helper function to create an event payload
 def create_event_payload(start_offset_days=0, end_offset_days=1):
-    start_time = datetime.utcnow() + timedelta(days=start_offset_days)
-    end_time = datetime.utcnow() + timedelta(days=end_offset_days)
+    # Ensure datetimes are timezone-aware (UTC)
+    start_time = datetime.now(timezone.utc) + timedelta(days=start_offset_days)
+    end_time = datetime.now(timezone.utc) + timedelta(days=end_offset_days)
     return {
         "title": fake.sentence(nb_words=3),
         "description": fake.text(),
-        "start": start_time.isoformat(),  # Rename start_time to start
-        "end": end_time.isoformat(),  # Rename end_time to end
+        "start": start_time.isoformat().replace("+00:00", "Z"),  # Ensure Z suffix
+        "end": end_time.isoformat().replace("+00:00", "Z"),  # Ensure Z suffix
         "all_day": fake.boolean(),
     }
 
@@ -31,13 +33,20 @@ def test_create_event_unauthorized(client: TestClient) -> None:
     assert response.status_code == status.HTTP_401_UNAUTHORIZED
 
 
-def test_create_event_success(db: Session, client: TestClient) -> None:
+def test_create_event_success(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:
     """Test creating a calendar event successfully."""
     user, password = generators.create_user(db)
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
     payload = create_event_payload()
 
+    # Mock the celery task sending
+    mock_send_task = mocker.patch(
+        "core.celery_app.celery_app.send_task"
+    )  # Corrected patch target
+
     response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
@@ -49,7 +58,7 @@ def test_create_event_success(db: Session, client: TestClient) -> None:
     data = response.json()
     assert data["title"] == payload["title"]
     assert data["description"] == payload["description"]
-    # Remove the '+ "Z"' as the API doesn't add it
+    # Assert with Z suffix
     assert data["start"] == payload["start"]
     assert data["end"] == payload["end"]
     assert data["all_day"] == payload["all_day"]
@@ -62,6 +71,11 @@ def test_create_event_success(db: Session, client: TestClient) -> None:
     assert event_in_db.user_id == user.id
     assert event_in_db.title == payload["title"]
 
+    # Assert that the task was called correctly
+    mock_send_task.assert_called_once_with(
+        "modules.calendar.tasks.schedule_event_notifications", args=[data["id"]]
+    )
+
 
 # --- Test Get Events ---
 
@@ -72,36 +86,49 @@ def test_get_events_unauthorized(client: TestClient) -> None:
     assert response.status_code == status.HTTP_401_UNAUTHORIZED
 
 
-def test_get_events_success(db: Session, client: TestClient) -> None:
+def test_get_events_success(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test getting all calendar events for a user."""
-    user, password = generators.create_user(db)
+    user, password = generators.create_user(
+        db, username="testuser_get_events"
+    )  # Unique username
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
 
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     # Create a couple of events for the user
     payload1 = create_event_payload(0, 1)
-    client.post(
+    create_rsp1 = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload1,
     )
+    assert create_rsp1.status_code == status.HTTP_201_CREATED
+
     payload2 = create_event_payload(2, 3)
-    client.post(
+    create_rsp2 = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload2,
     )
+    assert create_rsp2.status_code == status.HTTP_201_CREATED
 
     # Create an event for another user (should not be returned)
-    other_user, other_password = generators.create_user(db)
+    other_user, other_password = generators.create_user(
+        db, username="otheruser_get_events"
+    )  # Unique username
     other_login_rsp = generators.login(db, other_user.username, other_password)
     other_access_token = other_login_rsp["access_token"]
     other_payload = create_event_payload(4, 5)
-    client.post(
+    create_rsp_other = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {other_access_token}"},
         json=other_payload,
     )
+    assert create_rsp_other.status_code == status.HTTP_201_CREATED
 
     response = client.get(
         "/api/calendar/events", headers={"Authorization": f"Bearer {access_token}"}
@@ -115,35 +142,51 @@ def test_get_events_success(db: Session, client: TestClient) -> None:
     assert data[1]["user_id"] == user.id
 
 
-def test_get_events_filtered(db: Session, client: TestClient) -> None:
+def test_get_events_filtered(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test getting filtered calendar events for a user."""
-    user, password = generators.create_user(db)
+    user, password = generators.create_user(
+        db, username="testuser_filter_events"
+    )  # Unique username
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
 
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     # Create events
     payload1 = create_event_payload(0, 1)  # Today -> Tomorrow
-    client.post(
+    create_rsp1 = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload1,
     )
+    assert create_rsp1.status_code == status.HTTP_201_CREATED
+
     payload2 = create_event_payload(5, 6)  # In 5 days -> In 6 days
-    client.post(
+    create_rsp2 = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload2,
     )
+    assert create_rsp2.status_code == status.HTTP_201_CREATED
+
     payload3 = create_event_payload(10, 11)  # In 10 days -> In 11 days
-    client.post(
+    create_rsp3 = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload3,
     )
+    assert create_rsp3.status_code == status.HTTP_201_CREATED
 
     # Filter for events starting within the next week
-    start_filter = datetime.utcnow().isoformat()
-    end_filter = (datetime.utcnow() + timedelta(days=7)).isoformat()
+    start_filter = datetime.now(timezone.utc).isoformat().replace("+00:00", "Z")
+    end_filter = (
+        (datetime.now(timezone.utc) + timedelta(days=7))
+        .isoformat()
+        .replace("+00:00", "Z")
+    )
 
     response = client.get(
         "/api/calendar/events",
@@ -157,7 +200,11 @@ def test_get_events_filtered(db: Session, client: TestClient) -> None:
     assert data[1]["title"] == payload2["title"]
 
     # Filter for events starting after 8 days
-    start_filter_late = (datetime.utcnow() + timedelta(days=8)).isoformat()
+    start_filter_late = (
+        (datetime.now(timezone.utc) + timedelta(days=8))
+        .isoformat()
+        .replace("+00:00", "Z")
+    )
     response = client.get(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
@@ -172,34 +219,48 @@ def test_get_events_filtered(db: Session, client: TestClient) -> None:
 # --- Test Get Event By ID ---
 
 
-def test_get_event_by_id_unauthorized(db: Session, client: TestClient) -> None:
+def test_get_event_by_id_unauthorized(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test getting a specific event without authentication."""
     user, password = generators.create_user(db)
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload,
     )
+    assert create_response.status_code == status.HTTP_201_CREATED
     event_id = create_response.json()["id"]
 
     response = client.get(f"/api/calendar/events/{event_id}")
     assert response.status_code == status.HTTP_401_UNAUTHORIZED
 
 
-def test_get_event_by_id_success(db: Session, client: TestClient) -> None:
+def test_get_event_by_id_success(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test getting a specific event successfully."""
     user, password = generators.create_user(db)
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload,
     )
+    assert create_response.status_code == status.HTTP_201_CREATED
     event_id = create_response.json()["id"]
 
     response = client.get(
@@ -210,6 +271,9 @@ def test_get_event_by_id_success(db: Session, client: TestClient) -> None:
     data = response.json()
     assert data["id"] == event_id
     assert data["title"] == payload["title"]
+    # Assert datetime with Z suffix
+    assert data["start"] == payload["start"]
+    assert data["end"] == payload["end"]
     assert data["user_id"] == user.id
 
 
@@ -227,20 +291,31 @@ def test_get_event_by_id_not_found(db: Session, client: TestClient) -> None:
     assert response.status_code == status.HTTP_404_NOT_FOUND
 
 
-def test_get_event_by_id_forbidden(db: Session, client: TestClient) -> None:
+def test_get_event_by_id_forbidden(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test getting another user's event."""
-    user1, password_user1 = generators.create_user(db)
-    user2, password_user2 = generators.create_user(db)
+    user1, password_user1 = generators.create_user(
+        db, username="user1_forbidden_get"
+    )  # Unique username
+    user2, password_user2 = generators.create_user(
+        db, username="user2_forbidden_get"
+    )  # Unique username
 
     # Log in as user1 and create an event
     login_rsp1 = generators.login(db, user1.username, password_user1)
     access_token1 = login_rsp1["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token1}"},
         json=payload,
     )
+    assert create_response.status_code == status.HTTP_201_CREATED
     event_id = create_response.json()["id"]
 
     # Log in as user2 and try to get user1's event
@@ -259,17 +334,24 @@ def test_get_event_by_id_forbidden(db: Session, client: TestClient) -> None:
 # --- Test Update Event ---
 
 
-def test_update_event_unauthorized(db: Session, client: TestClient) -> None:
+def test_update_event_unauthorized(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test updating an event without authentication."""
     user, password = generators.create_user(db)
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload,
     )
+    assert create_response.status_code == status.HTTP_201_CREATED
     event_id = create_response.json()["id"]
     update_payload = {"title": "Updated Title"}
 
@@ -277,12 +359,20 @@ def test_update_event_unauthorized(db: Session, client: TestClient) -> None:
     assert response.status_code == status.HTTP_401_UNAUTHORIZED
 
 
-def test_update_event_success(db: Session, client: TestClient) -> None:
+def test_update_event_success(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test updating an event successfully."""
     user, password = generators.create_user(db)
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch(
+        "core.celery_app.celery_app.send_task", return_value=None
+    )  # Mock for creation
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
@@ -299,6 +389,13 @@ def test_update_event_success(db: Session, client: TestClient) -> None:
         "all_day": not payload["all_day"],  # Toggle all_day
     }
 
+    # Mock celery task for update (needs separate mock)
+    mock_send_task_update = mocker.patch(
+        "modules.calendar.service.celery_app.send_task"
+    )
+    # Mock cancel notifications as well, as it's called synchronously in the service
+    mocker.patch("modules.calendar.tasks.cancel_event_notifications")
+
     response = client.patch(
         f"/api/calendar/events/{event_id}",
         headers={"Authorization": f"Bearer {access_token}"},
@@ -310,7 +407,8 @@ def test_update_event_success(db: Session, client: TestClient) -> None:
     assert data["title"] == update_payload["title"]
     assert data["description"] == update_payload["description"]
     assert data["all_day"] == update_payload["all_day"]
-    assert data["start"] == payload["start"]  # Check correct field name 'start'
+    # Assert datetime with Z suffix
+    assert data["start"] == payload["start"]
     assert data["user_id"] == user.id
 
     # Verify in DB
@@ -320,6 +418,17 @@ def test_update_event_success(db: Session, client: TestClient) -> None:
     assert event_in_db.description == update_payload["description"]
     assert event_in_db.all_day == update_payload["all_day"]
 
+    # Assert that the update task was called correctly
+    mock_send_task_update.assert_called_once_with(
+        "modules.calendar.tasks.schedule_event_notifications", args=[event_id]
+    )
+    # Assert cancel was NOT called because update doesn't cancel
+    # mock_cancel_notifications.assert_not_called() # Update: cancel IS called in update path via re-schedule
+    # Actually, schedule_event_notifications calls cancel_event_notifications first.
+    # So we need to mock cancel_event_notifications called *within* schedule_event_notifications
+    # OR mock schedule_event_notifications itself. Let's stick to mocking send_task.
+    # The cancel mock added earlier handles the direct call in the service layer if any.
+
 
 def test_update_event_not_found(db: Session, client: TestClient) -> None:
     """Test updating a non-existent event."""
@@ -337,20 +446,31 @@ def test_update_event_not_found(db: Session, client: TestClient) -> None:
     assert response.status_code == status.HTTP_404_NOT_FOUND
 
 
-def test_update_event_forbidden(db: Session, client: TestClient) -> None:
+def test_update_event_forbidden(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test updating another user's event."""
-    user1, password_user1 = generators.create_user(db)
-    user2, password_user2 = generators.create_user(db)
+    user1, password_user1 = generators.create_user(
+        db, username="user1_forbidden_update"
+    )  # Unique username
+    user2, password_user2 = generators.create_user(
+        db, username="user2_forbidden_update"
+    )  # Unique username
 
     # Log in as user1 and create an event
     login_rsp1 = generators.login(db, user1.username, password_user1)
     access_token1 = login_rsp1["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token1}"},
         json=payload,
     )
+    assert create_response.status_code == status.HTTP_201_CREATED
     event_id = create_response.json()["id"]
 
     # Log in as user2 and try to update user1's event
@@ -371,29 +491,42 @@ def test_update_event_forbidden(db: Session, client: TestClient) -> None:
 # --- Test Delete Event ---
 
 
-def test_delete_event_unauthorized(db: Session, client: TestClient) -> None:
+def test_delete_event_unauthorized(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test deleting an event without authentication."""
     user, password = generators.create_user(db)
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
         json=payload,
     )
+    assert create_response.status_code == status.HTTP_201_CREATED
     event_id = create_response.json()["id"]
 
     response = client.delete(f"/api/calendar/events/{event_id}")
     assert response.status_code == status.HTTP_401_UNAUTHORIZED
 
 
-def test_delete_event_success(db: Session, client: TestClient) -> None:
+def test_delete_event_success(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:
     """Test deleting an event successfully."""
     user, password = generators.create_user(db)
     login_rsp = generators.login(db, user.username, password)
     access_token = login_rsp["access_token"]
     payload = create_event_payload()
+
+    # Mock the celery task sending for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token}"},
@@ -408,12 +541,20 @@ def test_delete_event_success(db: Session, client: TestClient) -> None:
     event_in_db = db.query(CalendarEvent).filter(CalendarEvent.id == event_id).first()
     assert event_in_db is not None
 
+    # Mock the cancel_event_notifications function to prevent Redis call
+    mock_cancel_notifications = mocker.patch(
+        "modules.calendar.service.cancel_event_notifications"  # Target the function as used in service.py
+    )
+
     response = client.delete(
         f"/api/calendar/events/{event_id}",
         headers={"Authorization": f"Bearer {access_token}"},
     )
     assert response.status_code == status.HTTP_204_NO_CONTENT
 
+    # Assert that cancel_event_notifications was called
+    mock_cancel_notifications.assert_called_once_with(event_id)
+
     # Verify event is deleted from DB
     event_in_db = db.query(CalendarEvent).filter(CalendarEvent.id == event_id).first()
     assert event_in_db is None
@@ -441,20 +582,31 @@ def test_delete_event_not_found(db: Session, client: TestClient) -> None:
     assert response.status_code == status.HTTP_404_NOT_FOUND
 
 
-def test_delete_event_forbidden(db: Session, client: TestClient) -> None:
+def test_delete_event_forbidden(
+    db: Session, client: TestClient, mocker: MockerFixture
+) -> None:  # Add mocker
     """Test deleting another user's event."""
-    user1, password_user1 = generators.create_user(db)
-    user2, password_user2 = generators.create_user(db)
+    user1, password_user1 = generators.create_user(
+        db, username="user1_forbidden_delete"
+    )  # Unique username
+    user2, password_user2 = generators.create_user(
+        db, username="user2_forbidden_delete"
+    )  # Unique username
 
     # Log in as user1 and create an event
     login_rsp1 = generators.login(db, user1.username, password_user1)
     access_token1 = login_rsp1["access_token"]
     payload = create_event_payload()
+
+    # Mock celery task for creation
+    mocker.patch("core.celery_app.celery_app.send_task")
+
     create_response = client.post(
         "/api/calendar/events",
         headers={"Authorization": f"Bearer {access_token1}"},
         json=payload,
     )
+    assert create_response.status_code == status.HTTP_201_CREATED
     event_id = create_response.json()["id"]
 
     # Log in as user2 and try to delete user1's event