Smith's Bible Dictionary
Pen'tecost. Pentecost, that is, the fiftieth day, (from a Greek word, meaning fiftieth), or Harvest Feast, or Feast of Weeks, may be regarded as a supplement to the Passover. It lasted for but one day. From the sixteenth of Nisan, seven weeks were reckoned inclusively, and the next or fiftieth day was the Day of Pentecost, which fell on the sixth of Sivan, (about the end of May). Exodus 23:16; Exodus 34:22; Leviticus 23:15; Leviticus 23:22; Numbers 28. See the Jewish Calendar at the end of this volume.
The Pentecost was the Jewish harvest-home, and the people were especially exhorted to rejoice before Jehovah with their families their servants, the Levite within their gates, the stranger, the fatherless and the widow, in the place chosen by God, for his name, as they brought a free-will offering, of their hand to Jehovah their God. Deuteronomy 16:10-11.
The great feature of the celebration was the presentation of the two loaves made from the first-fruits of the wheat harvest. With the loaves, two lambs were offered as a Peace Offering, and all were waved before Jehovah, and given to the priests; the leaves being leavened, could not be offered on the altar.
The other sacrifices were, a Burnt Offering of a young bullock, two rams, and seven lambs, with a meat and drink offering, and a kid for a Sin Offering. Leviticus 23:18-19. Till the Pentecostal, leaves were offered, the produce of the harvest might not be eaten, nor could any other firstfruits be offered. The whole ceremony was the completion, of that dedication of the harvest, to God as its giver, and to whom both the land, and the people were holy, which was begun by the offering of the wave-sheaf at the Passover. The interval is still regarded as a religious season.
The Pentecost is the only one of the three great feasts, which is not mentioned as the memorial of events, in the history of the Jews; but such a significance has been found in the fact, that the law was given from Sinai, on the fiftieth day after the deliverance from Egypt. Compare Exodus 12 and Exodus 19. In the Exodus, the people were offered to God as living first fruits; at Sinai, their consecration to him as a nation was completed. The typical significance of the Pentecost is made clear, from the events of the day recorded, in the Acts of the Apostles. Acts 2. Just as the appearance of God on Sinai was the birthday of the Jewish nation, so was the Pentecost, the birthday of the Christian Church.
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Smith, William, Dr. Entry for 'Pentecost'. Smith's Bible Dictionary. https://www.studylight.org/dictionaries/eng/sbd/p/pentecost.html. 1901.