On Sukkot, there was a special water pouring ceremony in the Temple in Jerusalem. In preparation for the ceremony, there were great celebrations in the Temple called Simchat Beit Hasho'eva, the Water Drawing Celebration. Though the Temple no longer stands and we no longer have the water pouring ceremony or the celebration, we still celebrate on Sukkot in commemoration. 

A fascinating discussion about the Water Pouring Ceremony and the Water Drawing Celebration.