diff --git a/app/api/routers/calendar.py b/app/api/routers/calendar.py
index 3af8c1d..da3fc82 100644
--- a/app/api/routers/calendar.py
+++ b/app/api/routers/calendar.py
@@ -4,6 +4,7 @@ BE-CAL-004: MinimumWorkload CRUD endpoints.
 BE-CAL-API-001: Single-slot creation endpoint.
 BE-CAL-API-002: Day-view calendar query endpoint.
 BE-CAL-API-003: Calendar slot edit endpoints (real + virtual).
+BE-CAL-API-004: Calendar slot cancel endpoints (real + virtual).
 """
 
 from datetime import date as date_type
@@ -23,6 +24,7 @@ from app.schemas.calendar import (
     MinimumWorkloadResponse,
     MinimumWorkloadUpdate,
     SlotConflictItem,
+    TimeSlotCancelResponse,
     TimeSlotCreate,
     TimeSlotCreateResponse,
     TimeSlotEdit,
@@ -44,6 +46,8 @@ from app.services.plan_slot import (
 )
 from app.services.slot_immutability import (
     ImmutableSlotError,
+    guard_cancel_real_slot,
+    guard_cancel_virtual_slot,
     guard_edit_real_slot,
     guard_edit_virtual_slot,
 )
@@ -442,6 +446,113 @@ def edit_virtual_slot(
     )
 
 
+# ---------------------------------------------------------------------------
+# Slot cancel (BE-CAL-API-004)
+# ---------------------------------------------------------------------------
+
+@router.post(
+    "/slots/{slot_id}/cancel",
+    response_model=TimeSlotCancelResponse,
+    summary="Cancel a real (materialized) calendar slot",
+)
+def cancel_real_slot(
+    slot_id: int,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Cancel an existing real (materialized) slot.
+
+    - **Immutability**: rejects cancellation of past slots.
+    - **Plan detach**: if the slot was materialized from a plan, cancelling
+      detaches it (sets ``plan_id`` to NULL) so the plan no longer claims
+      that date.
+    - **Status**: sets the slot status to ``Skipped``.
+    """
+    slot = (
+        db.query(TimeSlot)
+        .filter(TimeSlot.id == slot_id, TimeSlot.user_id == current_user.id)
+        .first()
+    )
+    if slot is None:
+        raise HTTPException(status_code=404, detail="Slot not found")
+
+    # --- Past-slot guard ---
+    try:
+        guard_cancel_real_slot(db, slot)
+    except ImmutableSlotError as e:
+        raise HTTPException(status_code=422, detail=str(e))
+
+    # --- Detach from plan if applicable ---
+    if slot.plan_id is not None:
+        detach_slot_from_plan(slot)
+
+    # --- Update status ---
+    slot.status = SlotStatus.SKIPPED
+    db.commit()
+    db.refresh(slot)
+
+    return TimeSlotCancelResponse(
+        slot=_slot_to_response(slot),
+        message="Slot cancelled successfully",
+    )
+
+
+@router.post(
+    "/slots/virtual/{virtual_id}/cancel",
+    response_model=TimeSlotCancelResponse,
+    summary="Cancel a virtual (plan-generated) calendar slot",
+)
+def cancel_virtual_slot(
+    virtual_id: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Cancel a virtual (plan-generated) slot.
+
+    This triggers **materialization**: the virtual slot is first converted
+    into a real TimeSlot row, then immediately set to ``Skipped`` status
+    and detached from its plan (``plan_id`` set to NULL).
+
+    - **Immutability**: rejects cancellation of past virtual slots.
+    """
+    # --- Validate virtual_id format ---
+    parsed = parse_virtual_slot_id(virtual_id)
+    if parsed is None:
+        raise HTTPException(status_code=400, detail=f"Invalid virtual slot id: {virtual_id}")
+
+    plan_id, slot_date = parsed
+
+    # --- Past-slot guard ---
+    try:
+        guard_cancel_virtual_slot(virtual_id)
+    except ImmutableSlotError as e:
+        raise HTTPException(status_code=422, detail=str(e))
+
+    # --- Materialize ---
+    try:
+        slot = materialize_from_virtual_id(db, virtual_id)
+    except ValueError as e:
+        raise HTTPException(status_code=404, detail=str(e))
+
+    # --- Verify ownership ---
+    if slot.user_id != current_user.id:
+        db.rollback()
+        raise HTTPException(status_code=404, detail="Slot not found")
+
+    # --- Detach from plan ---
+    detach_slot_from_plan(slot)
+
+    # --- Update status ---
+    slot.status = SlotStatus.SKIPPED
+    db.commit()
+    db.refresh(slot)
+
+    return TimeSlotCancelResponse(
+        slot=_slot_to_response(slot),
+        message="Virtual slot materialized and cancelled successfully",
+    )
+
+
 # ---------------------------------------------------------------------------
 # MinimumWorkload
 # ---------------------------------------------------------------------------
diff --git a/app/schemas/calendar.py b/app/schemas/calendar.py
index 607778b..4ed4582 100644
--- a/app/schemas/calendar.py
+++ b/app/schemas/calendar.py
@@ -4,6 +4,7 @@ BE-CAL-004: MinimumWorkload read/write schemas.
 BE-CAL-API-001: TimeSlot create / response schemas.
 BE-CAL-API-002: Calendar day-view query schemas.
 BE-CAL-API-003: TimeSlot edit schemas.
+BE-CAL-API-004: TimeSlot cancel schemas.
 """
 
 from __future__ import annotations
@@ -240,3 +241,13 @@ class CalendarDayResponse(BaseModel):
         default_factory=list,
         description="All slots for the day, sorted by scheduled_at ascending",
     )
+
+
+# ---------------------------------------------------------------------------
+# TimeSlot cancel (BE-CAL-API-004)
+# ---------------------------------------------------------------------------
+
+class TimeSlotCancelResponse(BaseModel):
+    """Response after cancelling a slot — includes the cancelled slot."""
+    slot: TimeSlotResponse
+    message: str = Field("Slot cancelled successfully", description="Human-readable result")