Queen Elizabeth II was born Elizabeth Alexandra Mary on April 21 1926 and so has already turned 90 years old.
But, since the mid-18th century, the monarch's birthday has been marked with the ceremony of the Trooping of the Colour.
That's all well and good when the monarch has a birthday in one of the warmer months of the year, but in the case of Edward VII, who came to the throne on Queen Victoria's death in 1901, everyone was likely to get a little wet and cold on parade on his birthday in November.
So he introduced an 'official' birthday at a more clement time of year.
George V was fine with a birthday in June but the Queen's father George VI had his birthday in December so he too adopted an official birthday in June.
When the Queen took the throne in 1952, she followed suit and her official birthday is held each year on a Saturday in June - usually the second.
This year that day is Saturday June 11, which is closer to Prince Philip's birthday.
He celebrated his 95th birthday on June 10.
As Britain's longest-reigning monarch, Queen Elizabeth II has been much photographed over the last nine decades.
In celebration of her 90th birthday, GetBucks combed its archives and put together 90 seconds of nostalgic photographs of The Queen as a celebration.
They range from a full family portrait of her christening to a photograph of Queen Elizabeth at St Paul's Cathedral on Friday June 10 2016 for a service to mark her official 90th birthday.
Many happy returns Your Majesty.