Wikipedia

Simchat Beit Hashoeivah or Simchas Beis Hashoeiva is a special celebration held by Jews during the Intermediate days of Sukkot
When the Temple in Jerusalem stood, a unique service was performed every morning throughout the Sukkot holiday: the Nisuch ha-Mayim (lit. "Pouring of the water") or Water Libation Ceremony. According to the Talmud, Sukkot is the time of year in which God judges the world for rainfall; therefore this ceremony, like the taking of the Four Species, invokes God's blessing for rain in its proper time. The water for the libation ceremony was drawn from the Pool of Siloam (Hebrew: Breikhat HaShiloah‎) in the City of David and carried up the Jerusalem pilgrim road to the Temple. The joy that accompanied this procedure was palpable. (This is the source for the verse in Isaiah: "And you shall draw waters with joy from the wells of salvation"