Why does she have two birthdays?

Queen Elizabeth II was born on 21 April 1926, but she has another official birthday, which is usually celebrated on the second Saturday of June.

So why two birthdays for the Queen and how does she celebrate them?

The double birthday tradition was started more than 250 years ago by King George II in 1748. He was born in November, which is not known in the UK for its good weather. But King George wanted it to be possible to have a big public celebration - and November wasn't the time do it. So, given that his actually birthday wouldn't be a good time of year for a birthday parade, he decided to combine it with an annual military parade in the summer, when the weather would hopefully be nice.

And so this is where the tradition of an official, public summer birthday for the monarch began!

How does she mark her birthdays?

The Queen usually spends her actual birthday with her family, although there will be special gun salutes in London at midday to mark it.

There will be a 41-gun salute in Hyde Park, a 21-gun salute in Windsor Great Park and a 62-gun salute at the Tower of London.

For her 'official' birthday' in June, the day is marked publicly with a big parade in London called Trooping the Colour.

Trooping the Colour has marked the official birthday of the British monarch for over 260 years. Over 1400 soldiers, 200 horses and 400 musicians take part in the event, so it's quite a spectacle! Lots of members of the public waving flags and wearing Union Jacks will fill the Mall outside Buckingham Palace to watch it.

On the day, a big parade will start at the Queen's official residence - Buckingham Palace - before moving along the Mall to Horse Guards Parade at Whitehall, near to Downing Street, and then back again.

This year will be the first time that newly-weds Prince Harry and Meghan Markle will be in on the balcony together, as husband and wife.

Source: www.bbc.co.uk/newsround