The word Songkran is from the Sanskrit language and means the passage of the sun from one sign of the Zodiac to another. There are twelve Songkrans each year, but this one (sometimes called Major Songkran) is when the sun enters the sign of Aries the Ram and is closely related to the ‘vernal’ or Spring Equinox - when the sun crosses the plane of the earth's equator, making night and day of approximately equal length all over the earth.
The day for celebrating Songkran varied with the equinox until 1888, when it was switched to a fixed date of 13 April, however in 1940, Thailand switched to officially celebrate New Year on January 1, in line with almost all other countries.
The Songkran celebration is similar to those of the Indian Holi Festival, the Chinese Ching Ming, and the Christian Festival of Easter. It is commonly believed that 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 Songkran is also a water festival, the throwing of water is meant to symbolize washing away back luck and sins from from the year before. April is also the hottest part of the year in Thailand, so being soaked is a refreshing escape from the heat and humidity. Nowadays Thai’s will walk the streets having water fights using containers of water, water guns, or stand at the side of roads with a hose and soak anyone who passes by... and in some towns and cities even the local fire brigades get involved in the celebrations using fire engines to hose down the crowds, to keep them cool in the heat.
You may also get covered in chalk, a custom originating from the chalk used by monks to mark blessings. This combination of water and powder is almost identical to the Indian Holi Festival, and it is possible that the customs originated in India.
Other customs are that anything old and useless must be thrown away so as not to bring bad luck to the owner and to make New Year resolutions.