The 26th of January ‘celebrates’ the arrival of The First Fleet in Sydney Cove in 1788, a group of 11 ships with a mixture of British military and predominantly White criminals and slaves with the aim of colonising Australia. It asserted Britain’s sovereignty and is basically the beginning date of White dominance over Indigenous Australians and their land.
It’s meant to be a day to celebrate diversity and ethnicity within the country, but it’s based on one of the biggest days in our history that illustrates intolerance and unacceptance for anyone that is perceived to be non-White.
Wikipedia covers it pretty well, and Chiara hits the nail on the head.