This celebration commenorates a speech given by the Buddha in which he laid down the first monastic regulations and predicted his own death. The speech was heard by 1250 enlightened monks who arrived without prior summons. The festival is to celebrate with chanting and presenting offering carious Vat throughout the country. The biggest celebrations take place at the Khmer ruins of Vat Phu Champasack.

Source: http://laos-adventures.com/page.asp?id=%7B4614FA47-8BAD-4899-80FF-9C9314BF0515%7D