The Hardest Day was a Second World War air battle fought on 18 August 1940 during the Battle of Britain between the German Luftwaffe and British Royal Air Force (RAF). On that day, the Luftwaffe made an all-out effort to destroy RAF Fighter Command. The air battles that took place on that day were amongst the largest aerial engagements in history to that time. Both sides suffered heavy losses. In the air, the British shot down twice as many Luftwaffe aircraft as they lost. However, many RAF aircraft were destroyed on the ground, equalising the total losses of both sides. Further large and costly aerial battles took place after 18 August, but both sides lost more aircraft combined on this day than at any other point during the campaign, including 15 September, the Battle of Britain Day, generally considered the climax of the fighting. For this reason, Sunday 18 August 1940 became known as "the Hardest Day" in Britain.

Zerstörergeschwader 26 "Horst Wessel" was a Luftwaffe heavy fighter wing of World War II.