Giving meat in due season (II Timothy 4:2), Jesus preached about the meaning of the Last Great Day, and His subject was the Holy Spirit. Why? There is no doubt that some understood the meaning of the day because His audience had just witnessed the conclusion of a ceremony that involved water. God never commanded them to keep this ceremony, but nonetheless it contained a measure of true symbolism.
Each day during the Feast of Tabernacles, a priest drew an urn of water from the pool of Siloam and carried it through the Water Gate while the people recited Isaiah 12:3: "Therefore with joy you will draw water from the wells of salvation." Once inside the city, they paraded the urn of water to the altar accompanied by a choir singing Psalms 113—118. To conclude the ritual, the priest poured the water on the altar as an offering to God.
However, on the last day, the great day of the Feast, they marched seven times around the altar before pouring the water. What does pouring water upon an altar have to do with salvation? How many understood the symbolism that day when Jesus spoke concerning the Holy Spirit? Had the symbolism become obscured in people's minds by the passage of time? Jesus' comment should have revitalized their understanding of this wonderful truth.
Psalm 118:19-29 is a part of what the choir was singing as the procession approached and circled the altar. This psalm exalts the theme of the Last Great Day. It depicts the time when the whole world will go through the gates of righteousness, recognizing Christ as Savior, rejoicing in those God sends to teach them and praising God for His mercy in giving them salvation. Though not directly stated in these verses, the only reason mankind will respond like this is because God will pour out His Holy Spirit on all of humanity!