Celebrating New Year at the time of the Vernal Equinox was very common in the past. The Songkran celebration is similar to those of the Indian Holi Festival, the Chinese Ching Ming, and the Christian Festival of Easter. Indeed April Fool's Day probably originated as mocking those who didn't accept the switch of New Year from April to January in France in the Sixteenth Century.
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.