On many Asian calendars, New Year's Day falls during the spring months. In Thailand, it's April 13, marking the start of a two-day festival called Songkran. Similar festivals, which follow the same calendar, occur under different names in Laos and Cambodia. Thais not only use this time to give their houses a good deep cleaning, but they also clean any images or statues of Buddha in their homes or at shrines. During parades, people may also throw water at images of Buddha to ritually cleanse them. Often, this water is mixed with perfume and fragrant herbs, and all of the cleanliness is supposed to bring blessings and good luck in the new year.
Originally, pouring water on others was meant to show them kindness and respect; water that had run off the Buddha images would be captured and then gently poured on elders and monks to bless and purify them. If you happen to be in Thailand during Songkran, though, you'd better be prepared to get a shower -- whether you want one or not. Eventually, the water purification ritual evolved to spraying people in the streets with hoses and squirt guns. Not exactly the original spirit of the whole thing, but considering how hot it is in April in Thailand (over 100 degrees Fahrenheit or 40 degrees Celsius), you might not mind.