Thai New Year (Songkran) is celebrated every mid-April, officially from April 13th to April 15th. Some parts of Thailand begin the festival on the evening of April 12th (eg. Patong Beach on Phuket Island).
Thai New Year is today a massive party for tourists and a reason for many to visit Thailand, specifically Koh Samui, Phuket and Pattaya.
Expect to get wet! Water-fights ensue for days and nights on end on almost every corner. Thais roam the streets with containers of water & water guns (sometimes mixed with talcum powder), or stand at the side of roads with a garden hose and drench each other and anyone walking past.
Bangla Road on Patong Beach in Phuket, is the best place to experience Thai New Year. With pubs and bars lining the streets, thousands of people equip themselves with water guns and spray anyone who comes in their path - no one is exempt!
There are even entire teams of partygoers with water tanks piled into the back of pickup trucks that make their way along the streets, throwing ice-cold water on anyone in their sight.