The Queen at 91: Why does she have two birthdays?
Queen Elizabeth II was born on 21 April 1926.
She's spending her 91st birthday in private at Windsor Castle.
But, the Queen also celebrates an 'official' birthday in the month of June.
That means she has two birthday celebrations a year.
Official celebrations to mark a King or Queen's birthday in the UK have often been held on a day that isn't their actual birthday.
Usually the official birthday happens in summer because there is a better chance of good weather in the UK during the summer months.
The two birthday tradition was started more than 250 years ago by King George II in 1748.
Trooping the Colour
On her official birthday, Her Majesty is joined by other members of the Royal Family at the Trooping the Colour parade.
Over 1400 officers and men on parade, 200 horses, and more than 400 musicians will take part in the event in London.
The parade starts at the Queen's official residence, Buckingham Palace, along The Mall to Horse Guards Parade, Whitehall, near to Downing Street, and back again.
This year Trooping the Colour takes place on Saturday 17 June.