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Pentecost means literally "the fiftieth day"
It was the second of the three great Jewish feasts, celebrated at Jerusalem yearly, the seventh week after the Passover and coincided with the end of wheat harvest.The institution of the feasts is recorded in Leviticus 23.
1st feast: Passover
Leviticus 23:4 ‘These are the feasts of the LORD, holy convocations (assemblies) which you shall proclaim at their appointed times.5 ‘On the fourteenth day of the first month at twilight is the LORD’S Passover.
On the day following the Passover Sabbath they were to bring a sheaf of firstfruits of their harvest as a wave offering. This would probably be a sheaf of Barley, as wheat harvest begins later in the month.
Leviticus 23:10... you shall bring a sheaf of the firstfruits of your harvest to the priest.11 ‘He shall wave the sheaf before the LORD, to be accepted on your behalf; on the day after the Sabbath the priest shall wave it.
Beginning on the day that they offered the wave offering of the sheaf they were to count 50 days.
Leviticus 23:15 ‘And you shall count for yourselves from the day after the Sabbath, from the day that you brought the sheaf of the wave offering: seven Sabbaths shall be completed.16 ‘Count fifty days to the day after the seventh Sabbath; then you shall offer a new grain offering to the LORD.
2nd feast: Firstfruits (called in the New Testament Pentecost)
Leviticus 23:17 ‘You shall bring from your dwellings two wave loaves of two-tenths of an ephah. They shall be of fine flour; they shall be baked with leaven. They are the firstfruits to the LORD.
The offering consisted of two loaves made from the firstfruits of the wheat harvest to which leaven (yeast) had been added.In the seventh month following the Day of Atonement on the tenth day came the Feast of Tabernacles.
Leviticus 23:27 "Also the tenth day of this seventh month shall be the Day of Atonement...
3rd feast: Tabernacles
Leviticus 23:34... ‘The fifteenth day of this seventh month shall be the Feast of Tabernacles for seven days to the LORD.
The Feast of Tabernacles came after the harvest of grapes, olives, dates, figs, pomegranates and other fruits, was finished.It becomes clear in the New Testament that these feasts had a prophetic significance.Christ was raised on the day following the Passover Sabbath indicating that the first sheaf of the harvest represented him.
1 Corinthians 15:20 But now Christ is risen from the dead, and has become the firstfruits of those who have fallen asleep.
From this it follows that the two leavened loaves offered at Pentecost almost certainly represent those that will be raised from the dead at his return.
1 Corinthians 15:23 But each one in his own order: Christ the firstfruits, afterward those who are Christ’s at His coming.
John 12:24 "Most assuredly, I say to you, unless a grain of wheat falls into the ground and dies, it remains alone; but if it dies, it produces much grain.
The feast of Tabernacles represents the complete harvest at the end of the millennial reign of Christ.
1 Corinthians 15:24 Then comes the end, when He delivers the kingdom to God the Father, when He puts an end to all rule and all authority and power.25 For He must reign till He has put all enemies under His feet.
26 The last enemy that will be destroyed is death.27 For "He has put all things under His feet." But when He says "all things are put under Him," it is evident that He who put all things under Him is excepted.28 Now when all things are made subject to Him, then the Son Himself will also be subject to Him who put all things under Him, that God may be all in all.
On the day of Pentecost, fifty days after Christ’s resurrection the Apostles were given a taste of the powers to come in the kingdom of God at Christ’s return.
Hebrews 6:5 and have tasted the good word of God and the powers of the age to come,
I hope this helps.