Australia is a member of the Commonwealth of Nations and the official head of state is the Monarch of the Realms. This is currently Queen Elizabeth II. She is also the head of state of many other countries and territories, including Great Britain, Northern Ireland, Canada and New Zealand. She was born on April 21, 1926, although her official birthday is celebrated on a range of dates in the different countries of the Commonwealth of Nations.
The Monarch's birthday was first celebrated in Australia in 1788. In that year, King George III was the Monarch and Governor Arthur Phillip declared a holiday to mark his birthday. Originally, the Monarch's birthday was celebrated on the anniversary of the actual date of birth of the King or Queen. However, after the death of King George V in 1936, the date remained close to his birthday, which was June 3.
So please make the most of this public holiday and stay safe & well.