Why do we commemorate the Battle of Britain on 15th September?

The 15th September is considered to be a pivotal point in the Battle of Britain.

Although other days during the Battle saw fiercer fighting and heavier losses, the 15th is the only day when two large daylight raids were made on London. German intelligence indicated that Fighter Command was on the brink of collapse and it was believed that the raids would provide the knock out blow needed to gain the air superiority sought by the Luftwaffe. Instead, the raids were convincingly met by the massed fighters of the RAF. The Luftwaffe lost 60 aircraft and morale was badly shaken.

RAF and WAAF staff plot Battle of Britain Raids
RAF and WAAF staff plot Battle of Britain Raids at RAF Uxbridge

Following the events on the 15th September the Luftwaffe switched to night bombing raids and Hitler postponed indefinitely the planned invasion of Great Britain. Although the Battle continued into October, the 15th, can be said to have been a decisive day in the conflict.