In fact, the holiday originated as a celebration of January 29, 1840, when William Hobson arrived in the Bay of Islands to proclaim the boundaries of New South Wales were to be extended to include the islands of New Zealand.
Initially, it was planned to declare January 29 a holiday every year, with festivities to be held on a Saturday. At some point however, it was "Mondayised" to create a three-day weekend.
Members of Hobson's team then sailed down the coast, arriving in Auckland on September 18, 1840. The group included Surveyor-General Felton Mathew and his wife Sarah. The preliminary agreement for the purchase of the site of Auckland was duly signed with Ngāti Whātua leaders the same day and the new settlement of Auckland was established as the capital of New Zealand.
While many of us associate Auckland Anniversary Day with the Auckland regatta, the first was actually held with masted ship and waka races on September 18. The first "Anniversary weekend" regatta was held on January 28, 1842.
However and wherever you're spending this weekend, enjoy.