The date was originally set by astrological calculations, but it is now fixed on 13 April. The festival may be extended if some of the celebrations fall over a weekend.
In Thailand, New Year is now officially celebrated on January 1, in line with almost all other countries. Songkran was the official New Year until 1888, when it was switched to a fixed date of 1 April. It wasn't until 1940, that this date was then shifted to 1 January.