A grand parade - Trooping of the Colour - marked the official birthday of the reigning British monarch.

After celebrating her 93rd birthday on April 21, a host of British royals celebrated Queen Elizabeth II's official birthday in London on Saturday.

So why exactly does Queen Elizabeth celebrate two birthdays?

Queen Elizabeth II was born on April 21, 1926. She celebrated her birthday earlier this year privately with the members of the royal family. According to People magazine, since her birthday also fell on Easter Sunday, she also attended the royal church service with her family.

And on Saturday, a grand parade known as the Trooping of the Colour marked the official birthday of the reigning British monarch.

According to royal regulations, it is customary to celebrate a sovereign's birthday publicly on a day during summer, preferably when the weather is pleasant.

It is believed that the tradition started during the reign of King George II in 1748. While the king was born on October, an annual 'Trooping of the Colour' became a celebration dedicated to the King, as well as the armed forces.