diff --git a/engine/apps/schedules/models/custom_on_call_shift.py b/engine/apps/schedules/models/custom_on_call_shift.py
index e39dd336..26ecc5b1 100644
--- a/engine/apps/schedules/models/custom_on_call_shift.py
+++ b/engine/apps/schedules/models/custom_on_call_shift.py
@@ -324,12 +324,14 @@ class CustomOnCallShift(models.Model):
                     break
                 last_start = start
                 day = CustomOnCallShift.ICAL_WEEKDAY_MAP[start.weekday()]
-                if (user_group_id, day, i) in combinations:
-                    all_rotations_checked = True
-                    break
+                # double-check day is valid (when until is set, we may get unexpected days)
+                if day in self.by_day:
+                    if (user_group_id, day, i) in combinations:
+                        all_rotations_checked = True
+                        break
 
-                starting_dates.append(start)
-                combinations.append((user_group_id, day, i))
+                    starting_dates.append(start)
+                    combinations.append((user_group_id, day, i))
                 # get next event date following the original rule
                 event_ical = self.generate_ical(start, 1, None, 1, time_zone, custom_rrule=day_by_day_rrule)
                 start = self.get_rotation_date(event_ical, get_next_date=True, interval=1)
@@ -386,7 +388,10 @@ class CustomOnCallShift(models.Model):
             if self.frequency is not None and self.by_day and start is not None:
                 start_day = CustomOnCallShift.ICAL_WEEKDAY_MAP[start.weekday()]
                 if start_day not in self.by_day:
-                    expected_start_day = min(CustomOnCallShift.ICAL_WEEKDAY_REVERSE_MAP[d] for d in self.by_day)
+                    # when calculating first start date, make sure to sort days using week_start
+                    sorted_days = [i % 7 for i in range(self.week_start, self.week_start + 7)]
+                    selected_days = [CustomOnCallShift.ICAL_WEEKDAY_REVERSE_MAP[d] for d in self.by_day]
+                    expected_start_day = [d for d in sorted_days if d in selected_days][0]
                     delta = (expected_start_day - start.weekday()) % 7
                     start = start + datetime.timedelta(days=delta)
 
diff --git a/engine/apps/schedules/tests/test_custom_on_call_shift.py b/engine/apps/schedules/tests/test_custom_on_call_shift.py
index 3dff4c72..b6c769b7 100644
--- a/engine/apps/schedules/tests/test_custom_on_call_shift.py
+++ b/engine/apps/schedules/tests/test_custom_on_call_shift.py
@@ -1712,3 +1712,80 @@ def test_until_rrule_must_be_utc(
     expected_rrule = f"RRULE:FREQ=WEEKLY;UNTIL={ical_rrule_until}Z;INTERVAL=4;WKST=SU"
 
     assert expected_rrule in ical_data
+
+
+@pytest.mark.django_db
+def test_week_start_changed_daily_shift(
+    make_organization_and_user,
+    make_schedule,
+    make_on_call_shift,
+):
+    organization, user_1 = make_organization_and_user()
+    schedule = make_schedule(organization, schedule_class=OnCallScheduleWeb, time_zone="Europe/Warsaw")
+
+    now = timezone.now().replace(hour=0, minute=0, second=0, microsecond=0)
+    today_weekday = now.weekday()
+    last_sunday = now - timezone.timedelta(days=7 + (today_weekday + 1) % 7)
+    last_saturday = last_sunday - timezone.timedelta(days=1)
+
+    # set week start to Sunday, so first event should be on last_sunday itself
+    data = {
+        "priority_level": 1,
+        "start": last_saturday,
+        "rotation_start": last_sunday,
+        "duration": timezone.timedelta(seconds=3600),
+        "frequency": CustomOnCallShift.FREQUENCY_DAILY,
+        "by_day": ["MO", "SU"],
+        "week_start": 5,  # SU
+        "interval": 1,
+        "schedule": schedule,
+    }
+    on_call_shift = make_on_call_shift(
+        organization=organization, shift_type=CustomOnCallShift.TYPE_ROLLING_USERS_EVENT, **data
+    )
+    rolling_users = [[user_1]]
+    on_call_shift.add_rolling_users(rolling_users)
+
+    ical_data = on_call_shift.convert_to_ical()
+    expected_start = "DTSTART;VALUE=DATE-TIME:{}T000000Z".format(last_sunday.strftime("%Y%m%d"))
+    assert expected_start in ical_data
+
+
+@pytest.mark.django_db
+def test_week_start_changed_daily_shift_until(
+    make_organization_and_user,
+    make_schedule,
+    make_on_call_shift,
+):
+    organization, user_1 = make_organization_and_user()
+    schedule = make_schedule(organization, schedule_class=OnCallScheduleWeb, time_zone="Europe/Warsaw")
+
+    now = timezone.now().replace(hour=0, minute=0, second=0, microsecond=0)
+    today_weekday = now.weekday()
+    last_sunday = now - timezone.timedelta(days=7 + (today_weekday + 1) % 7)
+    last_saturday = last_sunday - timezone.timedelta(days=1)
+    thursday = last_sunday + timezone.timedelta(days=4)
+
+    data = {
+        "priority_level": 1,
+        "start": last_saturday,
+        "rotation_start": last_sunday,
+        "duration": timezone.timedelta(seconds=3600),
+        "frequency": CustomOnCallShift.FREQUENCY_DAILY,
+        "by_day": ["MO", "SU"],
+        "week_start": 5,  # SU
+        "interval": 1,
+        "until": thursday,
+        "schedule": schedule,
+    }
+    on_call_shift = make_on_call_shift(
+        organization=organization, shift_type=CustomOnCallShift.TYPE_ROLLING_USERS_EVENT, **data
+    )
+    rolling_users = [[user_1]]
+    on_call_shift.add_rolling_users(rolling_users)
+
+    ical_data = on_call_shift.convert_to_ical()
+    # setting UNTIL to Thursday was generating extra events for current week Wednesday and Thursday
+    unexpected_by_days = ("BYDAY=WE", "BYDAY=TH")
+    for unexpected in unexpected_by_days:
+        assert unexpected not in ical_data