Why does the Queen have two birthdays?

This is common practice for monarchs who have not been born during the summer.

The Queen, like many other British monarchs through history, celebrates two birthdays a year.
Her actual birthday is on 21 April, but her official birthday is celebrated on a Saturday in June. This is similar to King Edward VII, who was born on 9 November, but celebrated his official birthday during either May or June.

This is common practice for monarchs who have not been born during the summer.The reasons for this is to assure a better chance of there being good weather for the Trooping the Colour parade which is held on the official birthday.

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This military ceremony was first held in the early eighteenth century where flags of the battalion were carried down the ranks so they could be recognised by the soldiers. So the real reason why her majesty has two birthdays? Tourism and the unpredictable British weather!