Bon Om Touk, or the Water Festival, celebrates the Tonle Sap River's annual change of direction in its water flow; this is the only waterway in the world that flows in opposite directions at different times of the year. The level of the river rises to such a degree at the end of the rainy season that water is forced into the Great Lake, Tonle Sap. This festival usually takes place at the end of October or beginning of November and lasts for three days. Most of the activities center around traditional boat races and firework displays. Each village has the opportunity to participate in the boat races, and boats are elaborately decorated and hand-carved to represent their respective villages. The boats are usually long canoes with as many as forty rowers. Millions of people from around the country head to Phnom Penh to watch the races and celebrate the festival, which is one of the most extravagant in the Khmer calendar.