The Queen celebrates two birthdays every year.
Queen Elizabeth's 84th birthday will be celebrated in Windsor this year with a bang.
The Queen's 'real' birthday celebrity will take place at the Long Walk near the entrance to Windsor Castle.
Windsor will celebrate the occasion with a traditional 21-gun salute.
The first cannon will be fired at midday.
The Queen's actual birthday is 21 April and is celebrated in private.
However, her official birthday is 12 June.
The tradition of two birthdays began with King Edward VII, born in November, who decided to celebrate a second birthday in June.
Queen Elizabeth's official birthdays were on the second Thursday of June for the first seven years of her reign.
It is now normally celebrated on Saturday.
Trooping of the Colour
Elizabeth Alexandra Mary Windsor's arrival in the world - officially timed at 0240 GMT on 21 April, 1926 - is toasted at parties in British embassies around the globe, usually at some time in June.
In Britain, the first Saturday in June is traditionally marked by the Trooping of the Colour ceremony in Horseguards' Parade.
For many years the Queen arrived at the party side-saddle on her favourite horse, Burmese.
Now the 84-year-old monarch is driven past the guard of honour in a horse-drawn carriage.
Her official birthday is also marked by the flying of the Union flag on government buildings, a 42-gun salute and the publication of the birthday honours list.
Queen's birthday worldwide
Australia, except for Western Australia, observes the Queen's Birthday on the second Monday in June.
In Western Australia, it is usually the last Monday of September or the first Monday of October.
The day has been celebrated since 1788.
The Queen's birthday is celebrated on the Monday before or on the 24 May. The day is known as Victoria Day, originally the birthday of Queen Victoria (born 24 May 1819).
The Queen's official birthday is marked by the firing of an artillery salute in the national and provincial capitals of Canada and the flying of the Union Flag on government buildings.
In New Zealand, the holiday is the first Monday in June. There have been proposals to replace the holiday with Matariki (M?ori New Year) as an official holiday.
The idea of renaming the Queen's birthday weekend to Hillary weekend, after Sir Edmund Hillary, the first person to ascend Mount Everest, was raised in 2009.
Gibraltar, Bermuda, Falklands, Hong Kong and Fiji
The Queen's official birthday is a public holiday in Gibraltar and most other British overseas territories, but in 2008, the Government of Bermuda decided that it would cease to be a public holiday in 2009, despite protests from people in the island.
In the Falkland Islands, the actual day of the Queen's birth, 21 April, is celebrated.
The day ceased to be a public holiday in Hong Kong after the territory's handover to the People's Republic of China in 1997.
Fiji also celebrates the Queen's official birthday, along with the Prince of Wales's birthday.
Although the Queen ceased to be head of state in 1987, she remains recognised by the Great Council of Chiefs as traditional Queen or paramount chief of Fiji.