from Wikipedia

Australia Day (previously known as Anniversary Day, Foundation Day and ANA Day) is the official 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 there, and the proclamation of British sovereignty over the eastern seaboard of New Holland.

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

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.