Songkran is the multi-day festival surrounding the Thai New Year; it came on Apr 13-15 this year. Although everyone is off from work and many stores are closed, the most visible evidence of Songkran is the giant 2-day water fight that occurs around the moat, the center of Chiang Mai. On the morning of the 13th, we piled some kids (including ours) into the back of a pickup and went off to get soaked.
Songkran is not something to enter lightly. No sir, you've got to be prepared. Undoubtedly the best way to prepare is the pickup truck. You find the biggest water containers you can, fill them up with water, buy whatever kind of squirtguns you prefer, grab a bunch of your friends, hop into the back of a pickup, and go cruise around the moat. The next best way is to do the same thing, only do it on the side of the road. While the pickup trucks and mopeds go by, you soak them. If you can't afford your own supply of water, no problem the moat is handy. Grab a bucket with a rope and you've got all the (very nasty) water you want. If by any chance you are a total slacker and forgot to come prepared, well, no problems, plenty of vendors are on hand to help you out.