Many anniversaries on this day according to Brian Swopes’ excellent This Day in Aviation site, including the first flights of the F-22 and the AH-1 Cobra.
For this Brit, however, 7th September refers to 7th September 1940, and the day the Luftwaffe campaign against the UK – aka the Battle of Britain – saw a very distinct change. As the Wikipedia article on the Blitz says: “From 7 September 1940, one year into the war, London was systematically bombed by the Luftwaffe for 57 consecutive nights. More than one million London houses were destroyed or damaged and more than 40,000 civilians were killed, almost half of them in London” A couple of memorable images were taken on September 7th, including one very iconic view of an He111 directly over the London docks.

A Heinkel He 111 flying over Wapping and the Isle of Dogs in the East End of London at at the start of the Luftwaffe’s evening raids of 7 September 1940. Taken from a German aircraft at 1848 hrs German time. (Imperial War Museum)