According to Wikipedia (with no citation, when I looked it up), the first British monarch to have an Official Birthday was George II, in 1748. Wikipedia doesn't give the actual date, but George II's actual birthday was 9 November (30 October in the Julian calendar) and the point of an Official Birthday has always been to celebrate it in summer.
The Trooping of the Colour was first staged in 1748, on George II's first official birthday, and has been held on the sovereign's Official Birthday ever since. It's officially known as the Sovereign's Birthday Parade.
Queen Elizabeth II's official birthday was 'normally' celebrated (in the UK, at least) on the second Saturday in June. (Neither Wikipedia nor the Royal website gives examples of any year when it was held on a different date.) Charles III's first two official birthdays were both celebrated on the third Saturday in June: 17 June 2023 and 15 June 2024.
Some other Commonwealth countries celebrate on different dates. In Canada, for example, Queen Victoria's birthday (24 May) was celebrated throughout the reign of Edward VIII (whose actual birthday was 9 November, the same day as George II's) but the birthdays of George V and Edward VIII were celebrated on their actual birthdays – both being in June (3rd and 23rd respectively). Canada celebrated the official birthday of George VI on various dates, between 20 May and 14 June, but since 1952 the sovereign's official birthday has been realigned with Victoria Day, and is now the last Monday before 25 May.
© Haydn Thompson 2024