From:
http://www.rafmuseum.org.uk/mileston...tary/1939.html 2 September 
The Advanced Air Striking Force (AASF) is deployed to France. The AASF initially
comprised twelve squadrons, ten equipped with Fairey Battle light bombers and two equipped with Hawker Hurricane single-seat fighters and is commanded by Air Vice Marshal P.H.L. Playfair.
3 September
At 1100hrs, an ultimatum issued to Germany by the British Government expires and in a radio address to the nation, Prime Minister Neville Chamberlain announces, "this Country is at war with Germany".
3 September
At 1130hrs, air-raid warning sirens sound in the London area for the first time. However, the warning is a false alarm, triggered by the detection of a French aircraft, en route to the United Kingdom, that had not filed a flight plan.
3 September
Australia, New Zealand and France declare war on Germany.
3 September
A Bristol Blenheim IV (N6215) of No.139 Squadron is the first Royal Air Force aircraft to cross the German frontier after war is declared. Between 1200hrs and 1650hrs the Blenheim, flown by Flying Officer A. McPherson, carries out a photographic and visual reconnaissance of German naval ports. Although the crew, which includes a naval observer, Commander Thompson, sight a number of warships in the Schillig Roads off Wilhelmshaven, their radio is unserviceable and they are unable to report until they return to Wyton. Flying Officer McPherson is subsequently awarded the Distinguished Flying Cross.
3 September
RAF Bomber Command conducts the Royal Air Force's first operational sorties of the Second World War, when eighteen Handley Page Hampdens and nine Vickers Wellingtons of RAF Bomber Command undertake a search for German naval shipping. However, they do not locate any targets and all return safely.
3 September
Pilot Officer John Noel Isaac of No.600 Squadron becomes the first Briton to die in the Second World War when his Bristol Blenheim crashes into Heading Street in Hendon at 1250hrs, 1 hour 50 minutes after the British declaration of war.
3-4 September
The first propaganda leaflet raid by Armstrong Whitworth Whitleys of No.51 and No.58 squadrons drops 5.4 million leaflets over targets included Hamburg, Bremen and the Ruhr.
4 September
A message from King George VI to the Royal Air Force on the outbreak of war:
"The RAF has behind it a tradition no less inspiring than those of the other Services and in campaigns which we have now been compelled to undertake you will have to assume responsibilities for greater than those which your Service had to shoulder in the last War. One of the greatest of them will be the safeguarding of this Island from the menace of the air. I can assure all ranks of the Air Force of my supreme confidence in their skill and courage and their ability to meet whatever calls may be made upon them."
4 September
Sergeant George Booth, an observer with No.107 Squadron becomes the first British Prisoner of War when his Bristol Blenheim is shot down over the German coast.
4 September
Following a second reconnaissance sortie by Flying Officer McPherson, fourteen Vickers Wellingtons of No.9 and No.149 Squadrons and fifteen Bristol Blenheims of Nos. 107, 110 and 139 Squadrons are despatched to attack German warships at Brunsbüttel, Wilhelmshaven and the Schillig Roads. However, five aircraft of each type do not locate their targets. Ten Blenheims from No.107 and No.110 Squadrons attack the pocket battleship Admiral Scheer and the cruiser Emden in Wilhemshaven. Three bombs hit the Scheer but fail to explode.
By a peculiar twist of fate a Blenheim piloted by Flying Officer H.L. Emden crashes on the deck of the cruiser of the same name. Two of the Wellingtons fail to find Brunsbüttel and bomb Esjberg in neutral Denmark, 110 miles north of their intended target. The error presaged the navigational difficulties which proved to be a consistent problem for Bomber Command. Flight Lieutenant K.C. Doran of No.110 Squadron is awarded the Distinguished Flying Cross for his part in this raid.
4 September
The first Royal Air Force attack on an enemy aircraft occurs when a Lockheed Hudson of No.224 Squadron fires on a Dornier Do18 over North Sea.
4 September
The British Expeditionary Force (BEF) begins to cross the English Channel to France with Air support provided by the Air Component of the BEF. This comprises four army co-operation squadrons, equipped with Westland Lysanders, four bomber squadrons equipped with Bristol Blenheims and four fighter squadrons equipped with Hawker Hurricanes and Gloster Gladiators.
5 September
An Avro Anson of No.500 Squadron makes the first attack on a German submarine, 10 miles north of Ostend.
6 September
The first German aircraft sorties occur over the United Kingdom.
6 September
South Africa declares war on Germany.
6 September
A technical fault at the Chain Home Radio Direction Finding (radar) station at Canewdon, compounded by a series of mistakes within RAF Fighter Command's fighter control system, leads to friendly aircraft being plotted as an incoming air raid.
No.56 Squadron scramble Hawker Hurricanes to intercept this 'phantom' raid and are plotted also as hostile. Further squadrons are scrambled and, tragically, a section of No.72 Squadron (Supermarine Spitfires) misidentifies two Hurricanes of No.56 Squadron as Messerschmitt Bf109s and shoots both aircraft down. One pilot, Pilot Officer M.L. Hulton-Harrop, is killed. The 'Battle of Barking Creek', as the events of 6 September later became known, led to a wholesale review of RAF Fighter Command's plotting system.
10 September
Canada declares war on Germany.
17 September
Soviet forces invade eastern Poland.
20 September
The first Women's Auxiliary Air Force plotters go on watch in Fighter Command Filter Rooms.
20 September
The first engagement between the Royal Air Force (RAF) and the Luftwaffe takes place, when three Fairey Battles of No.88 Squadron, Advanced Air Striking Force, are attacked by three Messerschmitt Bf109s of Jagdgruppe 152 west of Saarbrücken. Two Fairey Battles are shot down.