According to legend, this is to give thanks to their god, Kinoingan, for sacrificing his daughter, Huminodun, by cutting her to pieces and spread it across the field which then produced a fruitful harvest of the paddy. The word ‘Kaamatan’ means “harvest” in Kadazandusun.
The festival will last for the whole of the month of May, ending with a public holiday on a date selected by a priestess known as the bobohizan. A beauty pageant known as Unduk Ngadau will be held to commemorate the spirit of Huminodun, the mythological maiden who was of total beauty of the heart, mind and soul. The title is derived from the phrase runduk tadau which means “the girl crowned by the sunlight”. Unduk Ngadau Kaamatan is one of the most recognizable cultural events in Sabah.
There is also a dance performance called the Sumazau, a singing contest called Sugandoi, a bodybuilding competition, and other arts and crafts performances. Competitions such as hitting the gong and folk sports have also become one of the main events in this festival.
Popular drinks during the festival are tapai and Kinomol, which is a traditional alak drink. Tapai is drunk from a small bamboo vessel known as a sumbiling or from special glasses called singgarung, likewise made from bamboo.
According to Wikipedia and amazingborneo.com