diff --git a/backend/open_webui/migrations/versions/56359461a091_add_calendar_tables.py b/backend/open_webui/migrations/versions/56359461a091_add_calendar_tables.py
index 8277daa738..a0812578c8 100644
--- a/backend/open_webui/migrations/versions/56359461a091_add_calendar_tables.py
+++ b/backend/open_webui/migrations/versions/56359461a091_add_calendar_tables.py
@@ -24,7 +24,7 @@ def upgrade() -> None:
     sa.Column('user_id', sa.Text(), nullable=False),
     sa.Column('name', sa.Text(), nullable=False),
     sa.Column('color', sa.Text(), nullable=True),
-    sa.Column('is_system', sa.Boolean(), nullable=False),
+    sa.Column('is_default', sa.Boolean(), nullable=False),
     sa.Column('data', sa.JSON(), nullable=True),
     sa.Column('meta', sa.JSON(), nullable=True),
     sa.Column('created_at', sa.BigInteger(), nullable=False),
diff --git a/backend/open_webui/models/calendar.py b/backend/open_webui/models/calendar.py
index e055c87f90..859632c494 100644
--- a/backend/open_webui/models/calendar.py
+++ b/backend/open_webui/models/calendar.py
@@ -17,6 +17,7 @@ from sqlalchemy import (
     exists,
     func,
     delete,
+    update,
 )
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -40,7 +41,7 @@ class Calendar(Base):
     user_id = Column(Text, nullable=False)
     name = Column(Text, nullable=False)
     color = Column(Text, nullable=True)
-    is_system = Column(Boolean, nullable=False, default=False)
+    is_default = Column(Boolean, nullable=False, default=False)
     data = Column(JSON, nullable=True)
     meta = Column(JSON, nullable=True)
 
@@ -107,7 +108,8 @@ class CalendarModel(BaseModel):
     user_id: str
     name: str
     color: Optional[str] = None
-    is_system: bool = False
+    is_default: bool = False
+
     data: Optional[dict] = None
     meta: Optional[dict] = None
 
@@ -269,7 +271,7 @@ class CalendarTable:
                     user_id=user_id,
                     name='Personal',
                     color='#3b82f6',
-                    is_system=True,
+                    is_default=True,
                     created_at=now,
                     updated_at=now,
                 ),
@@ -278,7 +280,6 @@ class CalendarTable:
                     user_id=user_id,
                     name='Scheduled Tasks',
                     color='#8b5cf6',
-                    is_system=True,
                     created_at=now + 1,
                     updated_at=now + 1,
                 ),
@@ -338,7 +339,6 @@ class CalendarTable:
                 select(Calendar).filter(
                     Calendar.user_id == user_id,
                     Calendar.name == 'Scheduled Tasks',
-                    Calendar.is_system == True,
                 )
             )
             cal = result.scalars().first()
@@ -349,7 +349,6 @@ class CalendarTable:
                     select(Calendar).filter(
                         Calendar.user_id == user_id,
                         Calendar.name == 'Scheduled Tasks',
-                        Calendar.is_system == True,
                     )
                 )
                 cal = result.scalars().first()
@@ -366,7 +365,7 @@ class CalendarTable:
                 user_id=user_id,
                 name=form_data.name,
                 color=form_data.color,
-                is_system=False,
+                is_default=False,
                 data=form_data.data,
                 meta=form_data.meta,
                 created_at=now,
@@ -403,13 +402,36 @@ class CalendarTable:
             await db.commit()
             return await self._to_calendar_model(cal, db=db)
 
+    async def set_default_calendar(
+        self, user_id: str, calendar_id: str, db: Optional[AsyncSession] = None
+    ) -> Optional[CalendarModel]:
+        """Set a calendar as the user's default, clearing all others."""
+        async with get_async_db_context(db) as db:
+            # Clear all defaults for this user
+            await db.execute(
+                update(Calendar)
+                .where(Calendar.user_id == user_id, Calendar.is_default == True)
+                .values(is_default=False)
+            )
+            # Set the new default
+            result = await db.execute(
+                select(Calendar).filter(Calendar.id == calendar_id, Calendar.user_id == user_id)
+            )
+            cal = result.scalars().first()
+            if not cal:
+                return None
+            cal.is_default = True
+            cal.updated_at = int(time.time_ns())
+            await db.commit()
+            return await self._to_calendar_model(cal, db=db)
+
     async def delete_calendar_by_id(self, id: str, db: Optional[AsyncSession] = None) -> bool:
-        """Delete a non-system calendar. Cascades to events, attendees, and grants."""
+        """Delete a non-default calendar. Cascades to events, attendees, and grants."""
         try:
             async with get_async_db_context(db) as db:
                 result = await db.execute(select(Calendar).filter(Calendar.id == id))
                 cal = result.scalars().first()
-                if not cal or cal.is_system:
+                if not cal or cal.is_default:
                     return False
 
                 # Delete attendees for all events in this calendar
diff --git a/backend/open_webui/routers/calendar.py b/backend/open_webui/routers/calendar.py
index 4b052bd754..220edf853c 100644
--- a/backend/open_webui/routers/calendar.py
+++ b/backend/open_webui/routers/calendar.py
@@ -310,10 +310,16 @@ async def delete_calendar(calendar_id: str, user: UserModel = Depends(get_verifi
     if cal.user_id != user.id and user.role != 'admin':
         raise HTTPException(status_code=403, detail='Only owner can delete calendar')
 
-    if cal.is_system:
-        raise HTTPException(status_code=400, detail='Cannot delete system calendar')
 
     result = await Calendars.delete_calendar_by_id(calendar_id)
     if not result:
         raise HTTPException(status_code=500, detail='Failed to delete')
     return {'status': True}
+
+
+@router.post('/{calendar_id}/default')
+async def set_default_calendar(calendar_id: str, user: UserModel = Depends(get_verified_user)):
+    cal = await Calendars.set_default_calendar(user.id, calendar_id)
+    if not cal:
+        raise HTTPException(status_code=404, detail='Calendar not found')
+    return cal
diff --git a/src/lib/apis/calendar/index.ts b/src/lib/apis/calendar/index.ts
index 39540bb890..fa75f9a6a7 100644
--- a/src/lib/apis/calendar/index.ts
+++ b/src/lib/apis/calendar/index.ts
@@ -5,7 +5,7 @@ export type CalendarModel = {
 	user_id: string;
 	name: string;
 	color: string | null;
-	is_system: boolean;
+	is_default: boolean;
 	data: Record<string, any> | null;
 	meta: Record<string, any> | null;
 	access_grants: any[];
@@ -188,6 +188,37 @@ export const deleteCalendar = async (token: string, calendarId: string): Promise
 	return res?.status ?? false;
 };
 
+export const setDefaultCalendar = async (
+	token: string,
+	calendarId: string
+): Promise<CalendarModel> => {
+	let error = null;
+
+	const res = await fetch(`${WEBUI_API_BASE_URL}/calendars/${calendarId}/default`, {
+		method: 'POST',
+		headers: {
+			Accept: 'application/json',
+			'Content-Type': 'application/json',
+			authorization: `Bearer ${token}`
+		}
+	})
+		.then(async (res) => {
+			if (!res.ok) throw await res.json();
+			return res.json();
+		})
+		.catch((err) => {
+			error = err.detail;
+			console.error(err);
+			return null;
+		});
+
+	if (error) {
+		throw error;
+	}
+
+	return res;
+};
+
 // ── Events ─────────────────────────────────
 
 export const getCalendarEvents = async (
diff --git a/src/routes/(app)/calendar/+page.svelte b/src/routes/(app)/calendar/+page.svelte
index 89940ef23b..25671fc791 100644
--- a/src/routes/(app)/calendar/+page.svelte
+++ b/src/routes/(app)/calendar/+page.svelte
@@ -128,7 +128,7 @@
 		showEventModal = true;
 	}
 
-	$: defaultCalendarId = calendars.find((c) => !c.is_system || c.name === 'Personal')?.id || calendars[0]?.id || '';
+	$: defaultCalendarId = calendars.find((c) => c.is_default)?.id || calendars[0]?.id || '';
 
 	onMount(async () => {
 		await loadCalendars();