What does the annual Songkran water festival celebrate?

It is the biggest yearly festival in Thailand marking the Thai New Year. The festival takes place each year from April 13th to April 15th. The focus is on family making a clean start to the year and most importantly throwing water on just about everyone you can find! Wan Nao is the old traditional New Year's Eve on the night of April 14th. On the 15th it is traditional to go to the local Buddhist temple and build a sand castle in honor of the Buddha. It is worth noting that the Thai use a lunar calendar so the date is based on the full moon which is the middle of the month, so the night of April 14th would be the full moon.

https://www.marshall.edu/international/events/songkran-festival/