Battle.of Britain Day is celebrated annually on 15 September to commemorate the large-scale aerial battle of the same name, which took place on 15 September 1940, (German: Luftschlacht um England or Luftschlacht um Großbritannien). In Canada, the commemoration takes place on the third Sunday of September.
The background concerns Nazi Germany who had conquered most of Western Europe and Scandinavia by June 1940 and the British Empire and Commonwealth were the only major powers standing in the way of a German-dominated Europe. Adolf Hiltler, Chancellor of Nazi Germany attempted to appease the British however After having several peace offers rejected by the British, He ordered the Luftwaffe to destroy the Royal Air Force (RAF) in order to gain air superiority or air supremacy as a prelude to launching Operation Sea Lion, an amphibious assault by the Wehrmacht (German armed forces) onto the British mainland.
To prepare for this the Luftwaffe started attacking Merchant Shipping on the English Channel during July 1940 and the first Luftwaffe bomber fleets began attacking convoys and Royal Navy forces in English ports and Channel. The results were positive and the Germans succeeded in forcing the British to abandon the channel convoy route and to redirect shipping to ports in north-eastern Britain. With this achieved the Luftwaffe began the second phase of its air offensive, attacking RAF airfields and supporting structures on the British mainland. The codename of the offensive was Unternehmen Adlerangriff (“Operation Eagle Attack”). On 12 August, it flew its first missions in this regard. On 13 August, the Luftwaffe carried out its largest attack to date on the mainland. Christened Adlertag (“Eagle Day”), the attack was a failure. Nevertheless, the raids continued, at great cost to both sides. The impact of the German offensive on RAF airfields and Fighter Command is disputed. Some historians believe that the attacks were not having much effect and that the Germans were losing the attrition battle, while others believe the RAF was faltering.
However Hitler was dissatisfied with the progress being made. Prompted by an RAF raid on Berlin in late August 1940, he ordered the Luftwaffe to concentrate its attacks upon London. It was thought the move would draw RAF Fighter Command up into a large, decisive battle. Initially, the change in strategy caught the British off-guard. The first daylight attack of this type occurred on 7 September and caused extensive damage and civilian casualties. Some 107,400 long tons (109,100 t) of shipping was damaged in the Thames Estuary and 1,600 civilians were killed or injured. Hitler was dissatisfied with the Luftwaffe and its failure to destroy Fighter Command quickly. He dismissed over-optimistic reports from the Oberkommando der Luftwaffe (OKL or High Command of the Air Force), particularly the Chief of the Luftwaffe general staff Hans Jeschonnek, who asserted the RAF was on its last legs. Confident the RAF was nearly defeated, Jeschonnek requested terror bombing to be enacted as a final blow. Hitler refused, and only allowed attacks on industry, communications and public utility targets.
Over the next few days, bad weather prevented more large attacks. On 9 and 11 September, only smaller raids were carried out. It gave Hugh Dowding AOC (Air Officer Commanding) Fighter Command, the chance to prepare and reinforce his forces. The British, possibly through the use of Ultra intelligence, recognised the German change in strategy and duly prepared for further attacks on the capital although The intelligence from ULTRA at this stage in the war tended to be fragmented. Then in August, Operation Adlerangriff (Eagle Attack) was launched against RAF airfields in southern England. By the first week of September, the Luftwaffe had not gained the results desired by Hitler. Frustrated, the Germans turned towards the strategic bombing of cities, an offensive which was aimed at British military and civil industries, but also civilian morale. The attacks began on 7 September 1940, but were to reach their daylight climax on 15 September 1940 when the Luftwaffe launched its largest and most concentrated attack against London in the hope of drawing out the RAF into a battle of annihilation. Around 1,500 aircraft took part in the air battles which lasted until dusk. The action was the climax of the Battle of Britain during which RAF Fighter Command confronted the German raids. The Luftwaffe formations were dispersed by a large cloud base and failed to inflict severe damage on the city of London. In the aftermath of the raid, Hitler postponed Operation Sea Lion. Having been defeated in daylight, the Luftwaffe turned its attention to The Blitz night campaign which lasted until May 1941.