Well, low and behold, Google came to save the day;
“Australia Day is the official National Day of Australia. Celebrated annually on 26 January, it marks the anniversary of the 1788 arrival of the First Fleet of British ships at Port Jackson, New South Wales”
Sounds simple enough.
“In present-day Australia, celebrations reflect the diverse society and landscape of the nation and are marked by community and family events, reflections on Australian history”