Songkran is thailand’s traditional new year day and we have festival on 13 - 15 april.  
It’s probably the biggest festival in thailand.
 
http://www.bangkok.com/information-festivals/songkran.htm << this explains it better than I can.
 
http://en.wikipedia.org/wiki/Songkran_(Thailand)
 
 
The crime and accident rates are really high during the time. Lots of road and shop will close. Things are pretty chaotic. So you better plan and be careful if you intend to join the festival.
 
http://edition.cnn.com/2015/04/10/travel/songkran-water-festival-tips/
 
 
Thailand always ban wearing sexy and indecency on songkran every single year. It never stops people from doing so though - w-’  
Not to mention those super sexy night songkran party, those are pretty popular XD
 
I haven’t participate in the water festival for a long while now. It’s fun if you have the energy I guess. Theses day I just spend time with my family or go to temple.