According to the BBC, Queen Elizabeth has two birthdays as it’s more likely that the weather will be better for celebrations in the summer.
The queen isn’t the first royal in the family to do this. Per the British royal family’s website: “Official celebrations to mark the Sovereigns’ birthday have often been held on a day other than the actual birthday, particularly when the actual birthday has not been in the summer.”
Take, for example, her great-grandfather King Edward VII.
Source: https://www.distractify.com/p/why-does-the-queen-have-two-birthdays