January 26, 1788 was the day Captain Arthur Philip landed at Sydney Harbour and claimed New South Wales for King George III. So to indigenous people it is 'Invasion Day'.
http://en.wikipedia.org/wiki/Australia_day
I am a republican & would prefer the national day to be whenever we finally become a republic but that day seems long distant. In recent times the celebrations have become ugly & jingoistic with people draping themselves in the flag & getting drunk.