Fun fact, WW2 was meant to start on August 26th, 1939 as that was the original date set for the invasion of Poland, after Germany and the Soviet Union signed the German-Soviet Nonaggression Pact in Moscow on August 23rd,
Edit: The nonaggression pact was basically Germany aggreeing to give the Eastern two thirds of Poland to the USSR and keeping a third for themselves
But news of the signing of a treaty between Poland and Great Britain caused him to delay.
And on August 31st, 1939 he declared that the invasion should start at 4:45 the next morning, and on September 1st 1939 Germany invaded Poland.
Great Britain and France declared war on Germany two days later on September 3rd 1939 at 11:00 AM and 5:00 PM respectively
(Source: Encyclopedia Britannica)