The queen celebrates one of her birthdays on April 21st. Reuters

April 21 marks Queen Elizabeth’s official birthday and the longest-reigning monarch in the world will quietly celebrate her 91st birthday.

Yet, in June, the queen will also celebrate her birthday all over again. While Her Majesty was actually born on the April date, it’s a time-honored British royal tradition for a ruling king or queen to celebrate two birthdays in a calendar year.

The tradition was started by King George II in 1748 so that there would be even more reason to celebrate. A June date would ensure that no matter the monarch’s actual birthday, fair, pleasant weather would be promised for the big day. George chose to combine his new “birthday” with the spring military parade referred to as Trooping the Colour, where soldiers march with their colors for show, the Telegraph reported.

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For her 90th birthday last year, the queen enjoyed a large celebration with members of the royal family on the June date. The family watched the Trooping the Colour parade – made up of over 1,400 soldiers and men and about 200 hundred horses.

This year’s Trooping the Colour will take place on June 17, according to the Telegraph.

On her actual birthday in April, the queen met with residents on a “royal walkabout around Windsor,” according to the Washington Post. Her birthday was also marked with the lighting of over 1,000 torches and bonfires – a tradition on big occasions. She also met with former U.S. president Barack Obama and former first lady Michelle Obama.

On the evening of her actual birthday, her son Prince Charles hosted an intimate family dinner.