Today is a National Holiday on the Cayman Islands (where I live)...
HRH Queen Elizabeth II is not only the Queen of the UK but also the Head of the Commonwealth and is thus the Head of State in many other nations, nearly all of whom celebrate her status in some form or other.
One of the ways is to celebrate her birthday, either on the date of her birth, or through an "official birthday".
HRH Queen Elizabeth II was actually born on 21st April 1926, in London, the daughter of Prince Albert, Duke of York and The Duchess of York, the former Lady Elizabeth Bowes-Lyon. Her father became King George VI in December 1936 upon the abdication of his elder brother, King Edward VIII.
Queen Elizabeth II succeeded to the throne on the death of her father on 6th February 1952 and (as of January 2007) is the fourth longest reigning living Monarch in the world.
In a number of countries the Queen’s Birthday is celebrated as a Public Holiday on various other dates. For example; the Cayman Islands and Gibraltar celebrates the Queen's Birthday on the third Monday in June.
In all cases, the only "official" action which usually takes place on this day is the release of the names, under the Queen’s Birthday Honours system, of those citizens being honoured.
To me, the best way to celebrate her birthday is definitely with a Public Holiday, so Happy (non official) Birthday to you, Majesty.