The tradition of the monarch having multiple birthdays dates back to the days of Elizabeth I, who famously asked “what’s the use of being Queen if one can’t have as many birthdays as one desires?”
By the end of her reign, Elizabeth was having as many as four birthdays a year, often announced on the day if Her Majesty felt in need of a pick-me-up.
Because Elizabeth had created the Duke of Essex as Master of the Royal Extra-special Surprise Birthday (a birthday which was to fall on a different date each year, which Her Majesty was not to be informed of in advance, and which had extra cake), this tradition was carried on after her death.
During the Civil War, almost all of the royal birthdays were lost or destroyed by Parliamentary forces. The two surviving birthdays were concealed by Royalists in a disused privy at Edinburgh Castle, and were not celebrated again until after the Restoration.
The apparent multiplication of royal birthdays with the expansion of Empire is in fact an illusion brought on by one of the birthdays having crossed the International Date Line during World War II, when it was shipped to New Zealand for safe keeping.
In Australia in the 1990s, lukewarm enthusiasm for republicanism and the ban on fireworks lead to a brief-lived and unsuccessful movement to rechristen the Queen’s Birthday long weekend “It’s been a while since Easter”
Hope you enjoy your Queen(s’)(‘s) Birthdays(es)!