As we read on the Facebook profile of Ochotniczej Straży Pożarnej w Ocicach (Bolesławiec commune):
It was on 1 August 1944, at 5:00 p.m., that units of the Warsaw District of the Home Army and the operational units of the Home Army Headquarters attacked German facilities in all districts of occupied Warsaw, and the Warsaw Uprising began. That is why every year on 1 August, sirens can be heard to commemorate the fallen Warsaw Uprising insurgents. Precisely at 5:00 p.m.—at the “W” Hour.