Leviticus 23:15
And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete: And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete:
The count begins on the morrow after the Sabbath, on the day the sheaf of the wave offering is brought, and it continues through seven complete weeks. Counting fifty days, or seven weeks, from the offering of firstfruits at Passover time brings Pentecost. Pentecost is also called the feast of harvest and the feast of weeks. It lasts only one day, devoted to religious service.
At Pentecost the first fruits of the harvest were to be presented at the temple. Gratitude for grain prepared as food was expressed by presenting two loaves baked with leaven before God. That offering acknowledged dependence on God as the giver of all good and the duty to return gifts and offerings to support His cause.
On this appointed day the believers were the first fruits of the Christian church, and God poured out His Spirit upon them.