Songkran, the Thai New Year, is a fun holiday! It's a three-day festival in which people go crazy with a huge city-wide water fight! Songkran is a blast... for the first day or so.
By the third day, I usually experience a little "Songkran burnout". Getting splashed with water by random kids on the street looses some of its fun after three days, especially if all you want to do is walk to 7-11 for a snack.
Long-term expats know to hole up in their condos with Netflix and a case of Singha Beer for a few days.
Still, it's a fun time, and definitely something you shouldn't miss if you'll be in Thailand on April 13-15.