There are many advantages and responsibilities that come with being the reigning monarch of England, but one surprising perk is getting two birthdays each year. This year Saturday, June 11 marks the Queen’s official birthday, and will be celebrated around the Commonwealth. However, Elizabeth II was actually born on April 21.

The British monarch’s official birthday is not held on the same day each year, but instead is observed on a Saturday in June, usually the first or second weekend of the month. So why the moveable feast? Mostly because of the weather. The sovereign’s official birthday involves a lot of outdoor activities—such as the Trooping of the Colour military parade—and so the festivities were assigned a date when it was likely to be nice out. This shifted-birthday tradition dates back to 1748, when the annual summer military cavalcade became a celebration of the king as well as the armed forces—even though George II’s birthday was in October. Since then a monarch’s official birthday has generally been held in the summer, often quite removed from their actual day of birth. Elizabeth II’s great-grandfather Edward VII was born in November, but his official birthday celebration was always held in May or June. Elizabeth’s son Charles was also born in November and will most likely follow the convention when he ascends the throne.