Queen's Birthday in the UK (Flag Day) Date in the current year: April 21, 2017

Queen Elizabeth II celebrates her birthday twice. On the first, second, or third Saturday in June, the sovereign's official birthday is traditionally celebrated with various events. The Queen's actual birthday, April 21, is not a big holiday in the United Kingdom, but it still is an official flag day.
Elizabeth II (Elizabeth Alexandra Mary) was born on April 21, 1926 to Prince Albert, Duke of York (future King George VI) and Elizabeth, Duchess of York (future Queen Elizabeth the Queen Mother). In 1952, her father died and Elizabeth was proclaimed sovereign. Her coronation took place on June 2, 1953 and was the first coronation to be televised. As of 2014, Elizabeth II is the second longest-reigning British monarch. She has celebrated her Silver, Golden, and Diamond Jubilees in 1977, 2002, and 2012, respectively.
Birthdays of other members of the Royal Family are official flag days in the UK as well. They include (in chronological order) birthdays of Catherine, Duchess of Cambridge; Sophie, Countess of Wessex; Prince Andrew, Duke of York; Prince Edward, Earl of Wessex; Prince Philip, Duke of Edinburgh; Prince William, Duke of Cambridge; Camilla, Duchess of Cornwall; Anne, Princess Royal; Charles, Prince of Wales.