It's when the British go out to vote.
There is, however, no legal requirement for elections to be held on a Thursday.
It is simply convention.
The last time an election was held on another day was in 1931, when Britons trooped to the polls on a Tuesday.
Here are some other things you probably didn't know about general elections:
This election puts June equal top as the most popular month for choosing a new government since World War II.
Of the 15 general elections since 1945, four have now been held in June (1970, 1983, 1987 and 2001) and four in October (1951, 1959, 1964 and 1974).
Weather has no consideration for voters, and conditions have varied wildly over the years.
In 1964 much of the country was drenched, and there was some rain in all parts in February 1974, particularly in the south-west.
More recent elections have been dry and sunny, particularly in 1992 and 1997.
Few people thought this election would be held in June at all.
It was widely expected to be on 3 May, but was postponed because of the foot-and-mouth crisis.
Several previous governments have unexpectedly called off elections.
Sir Alec Douglas Home was widely expected to call an election in spring 1964, but after a Tory loss to Labour in the May Rutherglen by-election he postponed it until October that year.
In 1974 Ted Heath was expected to call the election on 7 February at the beginning of the miners strike and three-day week, but postponed it until 28 Feb and then lost.
James Callaghan - now Lord Callaghan of Cardiff - was expected to call an election in autumn 1978 following the collapse of the Lib-Lab pact when the 13 Liberal MPs withdrew their support.
But instead he called it for April, having lost a no-confidence motion following the infamous "Winter of Discontent".
There was speculation that John Major might call a "khaki election" in 1991, but having lost a succession of by-elections he did not call it until April 1992 - saying that it would have been 'immoral' to call it on the back of the Gulf War.
Unusually, this is the fourth time the whole of Wales has had the chance to go to the polls since the last general election.
There was the assembly referendum in 1997, and the Welsh Assembly and European Parliament elections in May and June 1999.
There has been only one Welsh by-election during this parliament - at Ceredigion in February last year.
The turnout in general elections since World War II has generally been around 75%.
The highest turnout since 1945 was in 1950, at 84%.
The lowest was in 1997, at 71.5%.
Although there is concern about falling turnout, that was only just above levels in 1970 and 1983.