Did you know that 26th January wasn't always called Australia Day? In New South Wales up until 1935, it was referred to as Anniversary Day, and the other states didn't celebrate the landing of the First Fleet in Sydney Cove until after Federation.
You can find more information at http://www.australiaday.org.au/australia-day/history/
This newspaper article from 26 January 1931 about Anniversary Day is also very interesting: http://trove.nla.gov.au/ndp/del/article/46595636
At the Sesquicentenary (150th anniversary) on 26 January 1938 in Sydney, there were not only celebrations but a conference was held for Aboriginal people who declared the day 'a Day of Mourning and Protest'.
There were big celebrations on Sydney Harbour for the Bicentenary in 1988, but there were also protests by Aboriginal people, many referring to the day as 'Invasion Day.'
While we have much to celebrate and be proud of in Australia, it must be very difficult for Aboriginal people, who are the first Australians, to be celebrating Australia Day on 26 January.