The actual day is the 26th, but if it occurs on a weekend, the public holiday is the following Monday.
To mark it, here is arguably the most visually appealing train in Australia's history, the Victorian Railways' Spirit of Progress. Introduced in November 1937, it ran as far as the NSW border station of Albury where because of the break of gauge, it had to stop. It was hauled by VR's three cylinder S class 4-6-2 steam locomotives, which typically had an eleven-car trailing load of 544 tons (553 tonnes).
In 1962 a standard gauge line was completed from Albury to Melbourne, so passenger trains to and from Sydney could travel right through; but by that stage steam had given way to diesel.