Queen's Birthday

The Queen's Official Birthday, or King's Official Birthday in the reign of a male monarch, is the selected day in some Commonwealth realms on which the birthday of the monarch (currently Queen Elizabeth II) is officially celebrated in those countries.

The sovereign's birthday was first officially marked in the United Kingdom in 1748, for King George II. Since then, the date of the king or queen's birthday has been determined throughout the British Empire, and later the Commonwealth of Nations, according to either different royal proclamations issued by the sovereign or governor or by statute laws passed by the local parliament. The date of the celebration today varies as adopted by each country and is generally set around the end of May or start of June, to coincide with a high probability of fine weather in the Northern Hemisphere for outdoor ceremonies, rather than with the monarch's actual birthday, that of the present monarch being 21 April. In some cases, it is an official public holiday, sometimes aligning with the celebration of other events. Most Commonwealth realms release a Queen's Birthday Honours list at this time.

Except in the states of Western Australia and Queensland, Australia observes the Queen's Birthday on the second Monday in June. Because Western Australia celebrates Western Australia Day (formerly known as Foundation Day) on the first Monday in June, the Governor of Western Australia each year proclaims the day on which the state will observe the Queen's Birthday, based on school terms and the Perth Royal Show. There is no firm rule to determine this date, though it is usually the last Monday of September or the first Monday of October. In 2012, Queensland celebrated the holiday in October, as the June holiday was reserved to mark Queen Elizabeth II's Diamond Jubilee as Queen of Australia, after which the holiday has reverted to its traditional date in line with the other eastern Australian states. Starting in 2016, Queensland celebrates the holiday on the first Monday of October, so that a long weekend coincides with the AFL and NRL grand finals.

...
Wikipedia