While we ordinary citizens only have one birthday a year, Queen Elizabeth II has two—actually, all of the British monarchs have two. Since 1748, sovereign’s birthdays have been officially celebrated on a specific day in June with the Queen’s (or King’s, depending on the monarch) Birthday Parade. The parade centers around a ceremony called Trooping the Colour*, featuring fully trained and operational troops from the Household Division. Given the British weather, the month of June was chosen to try to guarantee a beautiful day for the celebartions. So, even though Queen Elizabeth II was born on April 21, the official celebration of her birthday takes place either the first or second (or very rarely the third) Saturday in June—this year, it will be June 17. In some of the Commonwealth Realms the celebration is an official public holiday; however, that’s not the case in the monarch’s home, the United Kingom.
*In modern parlance, “trooping the colour” literally means "“carrying the flag,” a throwback to the early eighteenth century when the battalion colours were trooped down the ranks of soldiers so that they could be seen by all. Although the parade has marked the sovereign’s official birthday since 1748, it was only after Edward VII acceded to the throne in 1901 that the monarch started to take the salute in person.↩