On the last day of the feast Jesus made His most
important prophecy. John 7:37-38

Early in the morning the priest went to the pool
to draw water for the ceremonies at the temple.
Also on this day the people, after giving thanks
for the harvest, prayed for rain. I may have been
after the people had prayed that Jesus began to
speak of the life-giving water which He would give
to all those that believed on Him.

It was the prophecy of the coming of the Holy
Spirit, which would come upon the disciples on the
day of Pentecost.