The first monarch of all of Hawaii, Kamehaneha the Great united the island under one rule in the last part of the 1700s.
The day is marked by a call to the heritage of the islands and the original people. There are celebrations with traditional dance, food and parades. A center peice of the the celebration is the draping ceremony or laying on of flowers (Lei) on the statues of Kamehameha. This is done at a couple of different places, two in Hawaii and also the United States Capital in Washington DC (side note the only statue of a monarch in the Capital).
So have a great day!!!