The Water-Libation Ceremony
During the Feast of Tabernacles, the intense anticipation of rain came to be reflected in the temple services. Each morning of Tabernacles, a water libation (sacrificial pouring out of a liquid) was offered to the Lord as a visual prayer for rain. Shortly after dawn each morning, while the many sacrifices were being prepared, the high priest was accompanied by a joyous procession of music and worshipers down to the Pool of Siloam. The high priest carried a golden pitcher capable of holding a little more than a quart of water. He carefully dipped the pitcher into the pool and brought it back to the Temple Mount.
At the same time, another procession went down to a nearby location south of Jerusalem known as Motza where willows of the brook grew in great abundance. They they gathered the long, thin willows and brought them back to the temple. At the Temple, the willows were placed on the sides of the altar so that their tops formed a canopy of drooping branches over the altar. Meanwhile the high priest with the water from the Pool of Siloam had reached the southern gate of the Temple. It was known as the WATER GATE because of this ceremony. As he entered, three blasts of the silver trumpets sounded outside the Temple, and the priests with one voice repeated the words of Isaiah:
Isaiah 12:3 - Therefore with JOY you will draw water from the WELLS OF SALVATION
The high priest slowly proceeded then to the stone altar in the Inner Court of the Temple and ascended the right side of the ramp. At the peak, he turned to the left where there were two silver basins which drained to the base of the altar. One was reserved for the regular drink offerings (libations of wine) and one for the water libations during this feast. As the high priest raised the golden pitcher to pour out the water offering, the people shouted, "Raise your hand!" In response, the high priest lifted his hand higher and poured, allowing the people to verify his action. This tradition arose around 95 B.C. in response to an uprising in the days of Alexander Jannaeus, the king-priest grandson of Simon the Maccabees. The Maccabees were a family of priests led by their father, Matthias, who in 165 B.C. were instrumental in overthrowing Greco-Syrian rule of Israel. (More on this when we study Hanukkah). However, these descendants became a kingly line, wrongfully merging the offices of kings and priest. According to scripture, Israel's kings were to be from the tribe of Judah and lineage of David, and her priests from the tribe of Levi and lineage of Aaron. As a result, Jannaeus was hated by many of the Jewish brethren. As a Sadducee, he viewed the water pouring with contempt because it was not commanded by the Law of Moses but was a tradition. So instead of pouring the water into the basin, he poured it out upon the ground. The worshippers rioted, pelting him with citron fruits from the branches and sought to kill him. In a great rage, Alexander Jannaeus called the foreign missionary troops of his standing army to quell the riot. When the violent insurrection was finally subdued, six thousand people lay dead, and a horn was broken from the holy altar. Thereafter, the pouring ceremony was always closely scrutinized.
As the high priest poured out the water libation before the Lord, a drink offering of wine was simultaneously poured into the other basin. Three blasts of the silver trumpets immediately followed the pouring and signaled the start of the Temple music. The people listened as a choir of Levites sang the Hallel (i.e. the praise Psalms 113-118). At the proper time, the congregation waved their palm branches toward the altar and joined in singing: "Save now, I pray, Oh Lord; O Lord, I pray, send now prosperity" (Psalms 118:25). At the same time the priests, with palm branches in hand, marched once toward the altar.
Psalm 118 was viewed as a messianic psalm and as such gave the feast a messianic emphasis. This is why Jesus was greeted by the crowds shouting Hosanna (Hebrew for "SAVE NOW" in 118:25) and waving palm branches on His triumphal entry into Jerusalem (Matthew 21:8-9; Luke 19:38; John 12:13). They viewed Him as the Messiah King, come to deliver ("save now" Israel in fulfillment of Psalm 118.
Psalm 118:25 - Save now, I beseech thee, O Lord: O Lord, I beseech thee, send now prosperity.
Matthew 21:8-9 - And a very great multitude spread their garments in the way; others cut down branches from the trees, and strawed them in the way.  And the multitudes that went before, and that followed, cried, saying, Hosanna to the Son of David: Blessed is he that cometh in the name of the Lord; Hosanna in the highest.
Luke 19:38 - Saying, Blessed be the King that cometh in the name of the Lord: peace in heaven, and glory in the highest.
John 12:13 - Took branches of palm trees, and went forth to meet him, and cried, Hosanna: Blessed is the King of Israel that cometh in the name of the Lord.
They hailed Him with the messianic imagery of palm branches from the Feast of Tabernacles. This same imagery is in view in Revelation 7:9-10 where redeemed saints worship, with palm branches in hand, around the throne of God and of the Lamb:
Rev. 7:9-10 - After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;  And cried with a loud voice, saying, Salvation to our God which sitteth upon the throne, and unto the Lamb.
This custom of carrying branches and singing psalms during the Feast of Tabernacles is of ancient origin. It dates back at least to the time f the Maccabees, some 165 years before Christ (cf. Mac. 10:6-7). The water-drawing ceremony is also from antiquity. Although debate exists as to whether Isaiah alluded to the water-drawing ceremony (Isaiah 12:3) or the ceremony was derived from the words of Isaiah, it is known that the ceremony was in use at least 100 years before the time of Jesus.