Songkran (สงกรานต์) is the Thai New Year and the third New Year celebration we get every year* [*The first being International New Year celebrated on January 1st and the second being Chinese New Year normally some time in February.].
For a couple of days on the 13th and 14th of April every year the entire country goes crazy with water fights—in fact for many of the days before and after these dates too. The celebration comes at the end of the hot, dry season where temperatures can get to near 40 Celsius so the cooling water is always pretty welcome.
Although the most visible aspect of the festival is the huge water fights there is a more serious side to it as well. It is a time when the Buddha statues in homes and wats are taken out and cleaned and it is also a time that people show respect to their elders by accepting a blessing from them asked for through pouring water over their hands. This practice and that of monks blessing people by flicking holy water on them has been transformed into the water fights. Another practice of using plaster to mark a blessing has also been taken and Thais will mix plaster into the water and then smear that on others, again as a blessing.
There are always huge crowds numbered in the hundreds of thousands down in the Khao Sarn area of Bangkok where traffic is blocked for the two main days. In past years we have rented rooms at a hotel there and stayed for the main festival days. Always great fun!