The Queen's Official Birthday is the selected day in some Commonwealth realms on which the birthday of the monarch is officially celebrated in those countries. It does not necessarily correspond to the date of the monarch's actual birth.
The day is marked in London by the ceremony of Trooping the Colour, which is also known as the Queen's Birthday Parade. The list of Birthday Honours is also announced at the time of the Official Birthday celebrations.
The Sovereign's birthday was first officially marked in the Kingdom of Great Britain 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, either by Royal Proclamations issued by the sovereign or viceroy, 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 higher probability of fine weather in the Northern Hemisphere for outdoor ceremonies.