The Wheat Harvest is a celebration of the first fruits of the wheat harvest when two wave loaves of leavened bread were presented to the Lord in the temple. It is consider a shadow or type because 50 days after the resurrection of Jesus Christ who was anointed in heaven with the Holy Spirit as our High Priest as predicted by Aaron in the Old Testament.

Wheat harvest - Firstfruits presented to the Lord (Lev 23:17, 20)
Also called Feast of Weeks (Ex 34:22), the day of First Fruits (Exo 23:16, Num 28:26) Feast of the Harvest (Ex 23:16) and in the New Testament - Pentecost (Acts 2:1) A festival that celebrated the first fruits of the wheat harvest with the offering of two wave loaves of leavened bread (Lev 23:17, 20). This feast was also a shadow or type because fifty days after the resurrection, at the third hour morning offering at the temple (9 am - Acts 2:15), the firstfruits of the resurrection of saints on 16 Nisan were again presented by Jesus before the Father in heaven, and there was the first outpouring of the Holy Spirit, that resulted in three thousand souls being added to the church in one day (Acts 2:41), this rapid growth being symbolized by the loaves of leavened bread.
This was the day that Jesus was anointed in heaven with the Holy Spirit as our High Priest, which was also prefigured by the anointing of Aaron (Exo. 40:13) as mentioned in Psalms 133:1-3. Having finished His ministration in the outer court of the sanctuary (the work of justification by His sacrifice, Rom 5:9), Christ then began the priestly work of sanctification in the heavenly temple that was typified by the holy place, the first apartment of the earthly sanctuary. This outpouring of the Holy Spirit is also known as the early rain (Antitype) and reached from heaven down to the Apostles gathered in the upper room on mount Zion. There will also be a latter rain, the greater outpouring of the Holy Spirit near the end of time (Joel 2:23, Zech 10:1, James 5:7) for the proclamation of the three angels messages (Rev 14).
The Jews do not recognize the outpouring of the Holy Spirit in the second chapter of Acts as a fulfillment of Pentecost/Shavuot, so they are consequently unsure about the true prophetic meaning of the festival, commemorating instead the revelation of the Torah to Moses on Sinai.

Reference: The Biblical Feast Days. Retrieved on Feb. 29, 2016. http://biblelight.net/feasts.html