The Feast of Pentecost, or firstfruits, teaches us that Jesus Christ came to build His Church. This festival pictures the coming of the Holy Spirit and the establishment of the Church. The firstfruits are those who will be given salvation at the return of Christ. They have been empowered with the Holy Spirit, which creates in each one a new heart and nature to live by the commandments of God (Exodus 23:16 Exodus 23:16And the feast of harvest, the first fruits of your labors, which you have sown in the field: and the feast of ingathering, which is in the end of the year, when you have gathered in your labors out of the field.
American King James Version×; Acts 2:1-4 Acts 2:1-4 [1] And when the day of Pentecost was fully come, they were all with one accord in one place. [2] And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. [3] And there appeared to them cloven tongues like as of fire, and it sat on each of them. [4] And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
American King James Version×, Acts 2:37-39 Acts 2:37-39 [37] Now when they heard this, they were pricked in their heart, and said to Peter and to the rest of the apostles, Men and brothers, what shall we do? [38] Then Peter said to them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and you shall receive the gift of the Holy Ghost. [39] For the promise is to you, and to your children, and to all that are afar off, even as many as the LORD our God shall call.
American King James Version×; Acts 5:32 Acts 5:32And we are his witnesses of these things; and so is also the Holy Ghost, whom God has given to them that obey him.
American King James Version×; James 1:18 James 1:18Of his own will begat he us with the word of truth, that we should be a kind of first fruits of his creatures.
American King James Version×). Pentecost serves as a reminder that God grants His Holy Spirit to the firstfruits of His spiritual harvest.

To read more about Pentecost click the links below.

http://www.ucg.org/tags/pentecost

The Feast of Pentecost: The Firstfruits of God's Harvest

Why Should Christians celebrate the Feast of Pentecost?