Tabernacles was also the time when the Messiah was expected to appear and establish His Kingdom, and then God would dwell among them for ever. It was the only feast at which Gentiles were expected to worship in the coming Kingdom (Zechariah 14: 16,17).
During the festival, water was poured out symbolically in the temple, and became a particular focus for colourful, even riotous celebration. Israel depended on rain for its harvest, and so rejoiced in the rain given, and prayed for the needs of the coming year. But there was also an understanding of water as representing the Holy Spirit and the anticipation of a day when, according to the prophets, the Spirit would be poured out on all flesh (Isaiah 44:3, Joel 2:28,29). One of the scriptures traditionally sung was ‘Therefore with joy you will draw water from the wells of salvation’ (Isaiah 12:3), along with shouts of ‘Hosanna’, ‘Save now!’
Whoever believes in me, as Scripture has said, rivers of living water will flow from within them."
By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified. [John 7:37-39]