What is it about? First Fruits (Pentecost / Feast of Weeks) is calculated by counting 7 Sabbaths from the day after the Passover. The children of Israel would offer a free will offering unto the Most High according to how much he had blessed their harvest with increase. This was celebrated by offering the first fruits of all that was produced and bringing them to the Levites for a free will offering to the Most High. This was a time when all Israel would gather in Jerusalem to celebrate the Holy Convocation.
How do we celebrate it? Today we celebrate this by feasting and remembering the feast of First Fruits (Pentecost) in which the Disciples received the gift of the Holy Spirit and began to speak with different tongues and all men of all nations understood them as if they spoke in their native tongue in which they were born.
Act 20:16 For Paul had determined to sail by Ephesus, because he would not spend the time in Asia: for he hasted, if it were possible for him, to be at Jerusalem the day of Pentecost.
How Christ fulfilled this Holy Day?
After Christ was crucified, he promised that he would send a gift from on high to his disciples (Luke 24:49, John 14:16-17; 26, John 15:26). The disciples became the First fruits of Christ through receiving the Holy Spirit on the day of Pentecost (Acts 2).
Leviticus 23:9-22, Exodus 34:22, Exodus 23:16, Deuteronomy 16:9-12, Joshua 5:11, Acts 2:1-11, Act 20:16, Luke 24:49, John 14:16-17; 26, John 15:26, Acts 2nd chap