Pentecost is one of the three major Jewish festivals. It refers to the period 50 days after Passover. (Strictly speaking, the day after the first Sabbath following Passover is Firstfruits, the seven weeks including that Firstfruits is Feast of Weeks, and the 50th day after that is Pentecost.)
Lev 23:16Count fifty days to the day after the seventh Sabbath, and then present an offering of new grain to the Lord.Lev 23:17From wherever you live, bring two loaves made of two-tenths of an ephah of the finest flour, baked with yeast, as a wave offering of firstfruits to the Lord.Lev 23:18Present with this bread seven unblemished male lambs a year old, one young bull and two rams. They are to be a burnt offering to the Lord, together with their grain offerings and drink offerings—a food offering, an aroma pleasing to the Lord.Lev 23:19Then sacrifice one male goat for a sin offering and two male lambs a year old for a fellowship offering.Lev 23:20The priest is to wave the two lambs before the Lord as a wave offering, together with the bread of the firstfruits. They are a sacred offering to the Lord for the priest.Lev 23:21On that same day you are to proclaim a sacred assembly and do no regular work. This is to be a lasting ordinance for the generations to come, wherever you live.
This was also the day Moses received the Law on Mount Sinai. After escaping Egypt during Passover, they arrived at Mount Sinai around this time.
So, Pentecost is the day of harvest and the day the Law was received.
This Pentecost transitioned into the New Testament as the Feast of the Coming of the Holy Spirit. Fifty days after Jesus, the Passover Lamb, was crucified, the Holy Spirit descended upon the disciples on Pentecost, and the church was born.
Acts 2:1When the day of Pentecost came, they were all together in one place.Acts 2:2Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting.Acts 2:3They saw what seemed to be tongues of fire that separated and came to rest on each of them.Acts 2:4All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.Acts 2:5Now there were staying in Jerusalem God-fearing Jews from every nation under heaven.Acts 2:6When they heard this sound, a crowd came together in bewilderment, because each one heard their own language being spoken.