Did you know the Queen celebrates two birthdays? She gets to cut two cakes... and get two sets of presents!
The whole point is that in the case of British monarchs, two birthdays - an actual one and an official one - are commemorated. The difference is the official birthday is marked in case the actual day of birth falls outside the summer season, meaning inclement weather could hamper outdoor celebrations.
The Queen was born on 21 April - her actual birthday. Her official birthday, however, is usually a Saturday in June.
This is not a new tradition. King Edward VII, for example, was born on 9 November but his official birthday was marked, throughout his reign, as being in May or June, when there was greater likelihood of good weather for the birthday parade - also known as "Trooping the Colour", according to the official Web site of the monarchy.
The Queen usually spends her actual birthday privately, with family and friends. However, the occasion is marked by a 21-gun salute at Windsor Great Park, a 41-gun salute at Hyde Park and a 62-gun salute at the Tower of London. In addition, she does sometimes receive wishes from members of the public, as she did in 2006 when she met people on the streets outside her residence.
The official birthday, however, is celebrated in a big way - the highlight of which is the "Trooping the Colour" parade.
According to the official Web site, the parade is carried out by fully trained troops from the Household Division (Foot Guards and Household Cavalry) on Horse Guards Parade in Whitehall and is watched by members of the Royal Family, invited guests and members of the public. This military ceremony dates back, as far as is known, to the early 18th century, when colours (flags) of the battalion were carried (or "trooped") down the ranks so they could be seen and recognised by the soldiers.
Meanwhile, allow us to relive some more recent history.
The parade last year was a special one for Kate Middelton - it was her first as a member of the Royal Family. Incidentally, the parade itself was a landmark for the British monarchy - it was the first time three generations of the Royal family attended the event. The Queen wore a duck-egg blue coat for the event at which Lieutenant Tom Ogilvy, of the 1st Battalion Scots Guards, carried the honourable flag and "Les Huguenots" and "The Glaswegian" was played by the troops.
Also present at the event were the Duke of Edinburgh; the Prince of Wales and the Duchess of Cornwall; the Duke and Duchess of Cambridge; Prince Harry; the Princess Royal; the Duke of York and Princess Eugenie; and the Earl and Countess of Wessex, with their daughter Lady Louise.
The head of the monarchy takes the salute of the parade in person and that has been the custom, since the time of Edward VII. In addition, the sovereign also inspects the troops and at the end of the day, rides back to Buckingham Palace in a carriage.
This year, the "Trooping the Colours" parade will be held on 16 June, according to the official Web site.
Check Out BBC Archive Footage of the Queen (then Princess Elizabeth) Taking the Salute for the First Time in 1951: