It is fixed on these dates now. If these days fall on weekend the missed days off will be taken after the weekend - however originally its date was fixed by astrological calculation. Songkran falls in the hottest time of the year in Thailand, at the end of the dry season. Until 1888 the Thai New Year was the beginning of the year in Thailand, after using April 1 until 1940 now January 1 is the beginning of the year. The traditional Thai New Year is only a national holiday since then.
Originally this tradition started from the bathing ceremony, in which the Buddha images in the temples get cleaned. The young people poured small amount of scented water on the hands of their elders as a sign of respect, however nowadays this ritual changed from its traditional origins to just having fun.
New year traditions
The most significant tradition on Songkran is the throwing of water. Everybody meets on the streets with bowls of water, water guns or even a garden hose, and everyone passing by will be soaking wet quickly. Some even mix coloured powder into the water.Astrological calculation
Even though the traditional calendar of Thailand like most of Southeast Asia uses a lunisolar calendar, the date of the new year was calculated on a purely solar basis. The term Songkran comes from Sanskrit and means "a move or change" - in this case the move of the sun into the Aries zodiac. Originally this happened at the vernal equinox, but as the Thai astrology did not know about the precession the date moved from March to April.