The national dates for Songkran are April 13-15 and, at least in Chiangmai, those have been the fixed dates for years. I understand they celebrate it a few extra days in Pattaya and some rural areas (I've seen some water throwing still happening in small rural towns in the north after April 15th) but I understand (hopefully correctly) that the holiday is celebrated everywhere in Thailand between the 13th and 15th.