April 21st is the Queens birthday. She is actually very lucky because she has two birthdays.
Her actual birthday is on the 21st April but her official birthday is on a Saturday in June. For the reason, you have to go back to the reign of Edward VII (1901-1910). It had been the tradition since 1748 that the Monarch’s birthday had been celebrated by a ceremony called Trooping of the Colour, which takes place in Horse Guards Parade, not far from the Palace. However Edward VII was born on 9th November and it was thought that the weather could be too inclement to hold the ceremony then, so the ceremony for the monarchs birthday was moved to the second or third week in June, when it was thought that even the unreliable British weather would be better!
But did you know that Elizabeth was born in a relatively normal street in Mayfair and not in a palace like many of her predecessors?This was because her father (who became George VI) was not originally heir to the crown. That honour was reserved for his elder brother Edward VIII who of course abdicated in 1936 after his relationship with divorcee American Wallis Simpson forced him to give up the throne.
So outside the unlikely address of 17 Bruton St (now an expensive restaurant/night club) our future Queen was born by caeserean section at 2.40am, 21st April 1926.