One of the largest festivals of Cambodia revolves around the Tonle Sap Lake. The three-day Water Festival marks the changing of the flow of the Tonle Sap River and is also seen as thanksgiving to the Mekong River for providing the country with fertile land and abundant fish. It is celebrated in October or November depending on when the river flow reverts to its normal downstream direction (earlier during the rainy season the river reverses its course to an upstream direction). Then, as the rainy season tapers off and the swollen Tonle Sap Lake begins to empty back into the Mekong, the river changes direction once again, leaving behind vast quantities of fish.