Bonn Om Touk takes place once a year, on the full moon of the Buddhist month of Kadeuk (usually in November). The Water Festival lasts for 3 days around that time. The actual reversing of the river does not always happen at the time of the festival, although it does happen sometime around the end of the rainy season in November. Check future moon phases on moonconnection.com - See more at: http://www.somewhereintheworldtoday.com/festivals/bonn-om-touk/#sthash.BfUKhUjN.dpuf