from September 1939 through Man 1940. From this source:

"16 September 1939,
Britain starts convoy service for merchant shipping.
The British Carrier "Courageous", 22.500 tons, sunk by the U-29, German Kriegsmarine.
The Home Front during the Phoney War,
The first months of the war were an anti-climax for the majority of the British people. In anticipation of devastating air raids, children were hastily evacuated from expected target areas, a strict blackout was imposed and nearly all forms of popular entertainment were abruptly halted. When no great onslaught from the air occurred, relief led to a slackening of precautions. These quiet months from September 1939 until May 1940 were known as 'the phoney war.
About one and half million people, mostly children, were evacuated from the towns and cities to the countryside to avoid the expected air raids in the autumn of 1939. Many city children from poor parents found themselves in strange surroundings and their hosts were sometimes startled by the gulf between them. For instance, many children were unaccustomed to a proper bathroom in the house. The quiet of the early months of war meant that many mothers took their children home again. However, as the threatened air raids began later in 1940, a second wave of evacuees went to the country. Some parents, usually the more affluent, sent their children to friends and relatives overseas for the duration of the war. Canada and America were popular destinations."

The weeks following the end of the "Phoney War" were very intense.

"By Spring 1940, many people had decided that war was never going to happen, and they followed the advice of the newspaper headline which suggested: ‘Forget Hitler – take your holiday’. They stopped carrying their gas-masks. Six million people every night tuned in to listen to ‘Lord Haw-Haw’, the British Nazi who broadcast on the wireless from Germany...

... until, suddenly, on 9 April 1940, Nazi forces attacked Denmark and Norway."

Soon afterwards, they went through the rest of the countries on mainland Europe like crap through a goose.

It's hard to keep constantly alert, but this is no time to let our guard down.