The Commonwealth of Australia gained independence from the British Crown on the 3rd of September, 1939 through the Statute of Westminster Adoption Act 1942.
The act of declaring war on right-wing extremists and fascism in Germany, defending democracy, and protecting civilians from state-enacted genocide is the defining ethos of the modern and independent nation of Australia.
The third of September is an appropriate day to celebrate Australia as a modern democratic nation.
Here is a scan of the Act from https://www.foundingdocs.gov.au/item-sdid-96.html
