What is done to mark the occasion? Nothing much, apart from the release of the latest honours list.
At Blogocracy Tim Dunlop discusses the significance of the day and asks whether the ALP has a policy on the republic. It would seem not.
IMO we are already a de facto republic, with the PM as the President in all but name. The Governor General's role has been constrained in recent years. As his website attests, the GG is kept busy, but most of his duties are ceremonial, and not necessarily high profile ceremonial. That said, for now there doesn't seem to be much interest in the community, let alone the ALP, in changing our system of government, which means that the holiday is likely to stay.