On the 13th April the whole of Thailand celebrates Songkran - the Thai New Year. The celebrations take the form of a massive country-wide water fight. People take to the streets with water butts, hoses, buckets, water pistols or anything that can be used to soak passers by. A sight to be seen but no way to escape being soaked through several times whether you participate or not.
Aside from the informal celebrations, this year is Phi Phi's second official Songkran Festival, held near Cabana resort (left of the ferry pier). There will be Thai beauty and singing contests, a Grand Parade and of course water games! Kick off is around 4pm.
So whether you go to the bigger cities for the bigger celebrations or come to Phi Phi for something a little more local - be prepared to get a proper soaking.