As you can guess from the name, it is relaying events from WW II on the dates they happened, currently from the year 1940.
Over the last month or so, of course, it's been describing the German push through the Netherlands, Belgium, and through to Paris - for me, it brings home how desperate the situation must have looked from the UK at the time (and how incredible the evacuation at Dunkirk was).
Some examples of the tweets:
4 May - Surrounded by Germans after Allied retreat & down to 1 day's ammunition, 2,500 Norwegian troops- last in southern Norway- have surrendered.
10 May - 4.35AM German armies are now advancing into the Netherlands, Belgium, Luxembourg, & France. "Fall Gelb"- invasion of the west- has begun.
11 May - Germans invading Low Countries are just a feint. Main Wehrmacht force secretly advancing through Ardennes Forest, to trap Allies from rear.
15 May - Surrender of Dutch Army in Holland now being negotiated. Netherlands resisted for 5 days, at cost of 5000+ Dutch lives.
28 May - King Leopold has surrendered Belgium, unconditionally, to the Germans, saying: "The cause of the Allies is lost."
17 Jun - French Marshal Petain, already planning armistice with Germany: "Franco-British Union would be fusion with a corpse. Britain is doomed."
17 Jun - Noon. France's new PM, Marshal Pétain (84) on radio. "With a broken heart I tell you fighting must cease." French govt. seeking armistice.
Original post on Dreamwidth - there are comments there.