On this day, the last and greatest, there is a prayer called The Hosha' Na Rabba or The Great Salvation. Hosha' Na means "Save us Now" and that phrase is recited 7 times. It appears that this prayer was formed from the tradition that the Jewish people gathering in Jerusalem to celebrate the end of the harvest expected a great salvation to come at this time.
Once when Jesus celebrated Sukkot, in the midst of this last greatest day while the people were waiting expectantly, the true Messiah cried out with a loud voice, "Hey, if anyone is thirsty, let him come to Me and drink! Whoever believes in me, as the Scripture has said, streams of living water will flow from within Him."
That whole chapter people had been wondering who He was. Good man? Deceiver? Prophet? Messiah? Jesus removes the ambiguity for anyone... ANYONE... who would come and believe.
He is the Great Salvation! He is our Hope! He is our Messiah! Jesus! No one else.