The unique Songkran Festival in Chiang Mai lasts for 3 days. The first day (April 13 - Wan Sangkhan Long) is supposed to be the time for people to have their body, mind, home and surroundings cleaned. April 14 is called “Wan Nao”, which means a day of making merit in various ways; such as, offer sand to the temples, build Chedis, and conservative style of water splashing. April 15 or “Wan Phra Ya Wan” is a time for people to listen to the monk’s sermon, give wooden poles to support the great Bo tree, perform a bathing rite for Buddha images and monks, and enshrine the ashes of the ancestors, release birds and fish, pay respect to elders and ask for their blessing, as well as splash water on each other. During that time, the city area will be decorated with Lanna style arts for welcoming the event.

Ref.

http://www.tourismthailand.org