The Queen's Official Birthday (King's Official Birthday in the reign of a male monarch) is the selected day on which the birthday of the monarch of the Commonwealth realms (currently Queen Elizabeth II) is officially celebrated in those countries and in Fiji, which is now a republic. The date varies as adopted by each Commonwealth country, but is generally around the end of May to the start of June, to coincide with a high probability of fine weather in the Northern Hemisphere for outdoor ceremonies.

The sovereign's birthday was first officially marked in the United Kingdom in 1748. Since then, the date of the king or queen's birthday has been determined throughout the British Empire and later the Commonwealth according to either different royal proclamations issued by the sovereign or governor or by statute laws passed by the local parliament. The exact date of the celebration today varies from country to country and except by coincidence does not fall on the day of the monarch's actual birthday (that of the present monarch being 21 April^[1]). In some cases, it is an official public holiday, sometimes coinciding with the celebration of other events. Most Commonwealth realms release a Birthday Honours List at this time.