The Hoshana Rabbah Ceremony occurs on the 7th day of the Feast of Sukkot. It is on this day that God declares whether there will be rain for the coming year.
Everyday during the feast, the trumpets give three blasts, but on Hoshana Rabbah, the trumpets do three sets of seven blasts.
Every day during the feast, the priest went around the altar one time. On Hoshana Rabbah, the priest marches around the altar seven times.
As they march around the altar, they sing the Hosanna ("Save us") verse from Psalm 118:25 as the people wave their palm branches (the lulav).
This is why it is called the "Great Hosanna" (Hoshanna Rabbah) or the "Great Day of the Feast". This is when the prayers for water and the Messiah are at its peak.
This is when we see Jesus entering the scene. He declares loudly from amongst the crowd, "If anyone thirsts, let him come to me and drink. He who believes in me, as the scriptures say, out of his heart will flow rivers of living water." John 7:37-38; John 4:14
The priests were furious that the ceremony had been interrupted by the guy they had previously ordered to be arrested (John 7;14; 30-32).
According to the ancient writings, Hoshana Rabbah closes the period of divine judgment started on Rosh Hashana (the day the verdict was finalized). On Hoshana Rabbah, the verdict is sent out to the angels to perform it. The verdict for how blessed you will be and how much rain you will get this next year is signed, sealed and now delivered on this day.