Australia Day (previously known as Anniversary Day, Foundation Day and ANA Day) is the official national day (National Day) of Australia. Celebrated annually on 26 January, the day commemorates the arrival of the First Fleet at Sydney Cove in 1788, the hoisting of the British flag (Union Flag#History) there, and the proclamation of British sovereignty over the eastern seaboard of Australia.

Australia Day is an official public holiday (Holiday) in every state and territory (States and territories of Australia) of Australia, and is marked by the Order of Australia and Australian of the Year awards, along with an address from the Prime Minister (Prime Minister of Australia).

Although it was not known as Australia Day until over a century later, records of celebrations on 26 January date back to 1808, with Governor Lachlan Macquarie having held the first official celebration of the formation of New South Wales in 1818. In 2004, an estimated 7.5 million people attended Australia Day celebrations and functions across the country.

The day is seen as controversial for many Australians, particularly Indigenous Australians (Australian Aborigines), who see commemorating the arrival of the First Fleet as celebrating the destruction of the native Aboriginal culture by British colonists (British colonialism). Dating back to the 1938 Day of Mourning, there have been significant protests from and on behalf of the Indigenous Australian community, and the birth of the alternative name Invasion Day. Others have begun to use the name Survival Day to highlight that a people and culture expected to die out has survived. In light of these (and other) concerns, proposals to change the date of Australia Day have been made, but have failed to gain widespread public support.