The last day of the feast of Tabernacles would be the eighth day (cf. Lev. 23:36). Tradition records that during the first seven days of this feast it was customary for the priests and people to joyfully go to the pool of Siloam with a golden pitcher and bring back water to pour out before the altar. This was done to commemorate the time when Moses brought forth water from the rock (cf. Exo. 17:5-7).
On this occasion, Jesus once again shouted His message out (cf. John 7:28) - "If anyone thirsts, let him come to Me and drink" (7:37). If the above tradition is true, then it seems very likely that Jesus' words here were carefully chosen to draw a contrast between earthly water that ceases and "spiritual water" that endures (cf. 4:14; 6:35). Jesus invited the people to come to Him and receive this living water.
Jesus continued - "He who believes in Me, as the Scripture has said, out of his heart will flow rivers of living water" (7:38). Our Lord likely had the following passages in mind: Isaiah 44:3; 58:11; Zechariah 14:8. To go to Jesus and drink is to embrace His teachings to the fullest extent (i.e., to believe and obey them).
"But this He spoke concerning the Spirit, whom those believing in Him would receive; for the Holy Spirit was not yet given, because Jesus was not yet glorified" (John 7:39). Here is John's inspired interpretation of Jesus' words. John states that the time in which living water would flow like rivers from the hearts of men was not yet present. The time would not be ready until the Holy Spirit was "given," and this would not take place until after Jesus was "glorified." Certainly John is referring to Jesus' ascension where He would be restored to His position of honor that He had relinquished when He entered this world as a human being (cf. Phil. 2:5-11). And, according to Acts 1 & 2, after His ascension Jesus sent (or "gave") the Holy Spirit to the apostles (cf. Acts 2:4,33; John 16:7; Eph. 4:8) who in turn promised the "gift of the Holy Spirit" to all who would believe, repent, and be baptized (Acts 2:38,39).
A careful consideration of this context seems to support the conclusion that there is a difference between (1) the "living water" that refreshes the thirsty soul and gives salvation and (2) the flowing out of this water to others. The reason for making such a distinction is as follows: Surely Jesus did not invite spiritually thirsty people to come to Him to have their thirst quenched and then tell them that they would have to wait until later before such could be accomplished! No, the blessing of John 7:37 was available immediately (and is still available) through an obedient faith in Jesus. Anyone can and should come to Jesus and drink! However, on the Day of Pentecost after Jesus ascended into heaven, He made it possible for these living waters to flow out of others via the Holy Spirit and thereby satisfy thirsty souls who would receive and obey the truth.
Although this is admittedly a difficult passage, the application for us today is simple. One must develop a thirst for the truth (cf. Matt. 5:6) and then come to Jesus and drink by believing in Him and following His teachings. Those who believe that Jesus is the Messiah will be a blessing to others as they share the living water through the message of God's word that has been revealed by the Holy Spirit.