The celebration of the Thai New Year is a thousand year old tradition, celebrating last years harvest, It's a huge water throwing feast, across the whole country. The later years the party has developed, and the former rules, like stopping at 5 pm is no longer certain, so if you do not like to be wet, you should stay away between 13 and 16 April.
I myself, love the festival, even though i wish the "the rule" about stopping at 6pm would apply, cause it would be nice to know that you were safe after changing out of the wet clothes to go to dinner.
But its a great chance for you, to become a kid again, even just for 3 days..
P.S. at Ayutthaya, you are pretty much safe at the temple area, but downtown they even use elephants to spray you :-)
