From wikipedia: “The Songkran festival is celebrated in Thailand as the traditional New Year’s Day from 13 to 15 April. It coincides with the New Year of many calendars of South and Southeast Asia.”.
The throwing of water originated as a way to pay respect to people, by capturing the water after it had been poured over the Buddhas for cleansing and then using this “blessed” water to give good fortune to elders and family by gently pouring it on the shoulder.
Nowadays, it’s mainly a giant water-fight. It’s just crazy – even policemen can be soaked. Everybody is in – people on motorcycles actually stop in a polite way for allowing others to pour buckets of water onto them. Foreigners riding the tuk tuk in Bangkok are also excellent targets – I know that from experience. But it’s ok – April is one of the warmest months in Thailand, and it actually feels good getting a refreshing bucket of water onto you in the middle of the day.
As I don’t have underwater filming equipment, I dared not to take pictures, but here’s a video from youtube showing how fun it is: