In the Old Testament days, the day of Pentecost (the Pentecostal festival) was celebrated by the Israelites as the festival of harvest (the feast of harvest) or the feast of weeks, of the first-fruits of wheat harvest or the day of the first-fruits. This festival was celebrated after the feast of Passover. This festival day was calculated in the following way:
from the first Sunday (the first day of the week) after the feast of Passover, they counted seven full weeks (49 days). Seven full weeks will end on a Sabbath day or on a Saturday (the last day of the week). Then on the next day, the fiftieth day they celebrated the festival of harvest on Sunday (the first day of the week). See Exodus 23:16; 34:22; Numbers 28:26; Deuteronomy 16:10; and Leviticus 23:10-11, 14-17.
Since this festival of harvest was celebrated on the fiftieth day, it was called the day of Pentecost. In Greek, the day of Pentecost means the fiftieth day. Now in the New Testament period, the festival of harvest is celebrated as the festival of Pentecost or the Pentecostal festival. This festival is celebrated on the fiftieth day of our LORD JESUS CHRIST’s resurrection.