"Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD." (Leviticus 23:16)
This commandment represents the initiation of the Jewish Feast of Pentecost ("fiftieth day") which, many years later, was the day on which the Holy Spirit came to the church waiting in the upper room (Acts 2:1-4). There were seven such annual "feasts of Jehovah," all outlined in Leviticus 23, beginning with the Passover, commemorating the deliverance from Egypt, and culminating in the Feast of Tabernacles, in memory of their entrance into the promised land after dwelling in tents in the wilderness.
The middle feast of the seven was Pentecost, which seems to have been the anniversary of the giving of the law on Mount Sinai. It was scheduled 50 days after the "morrow after the sabbath" of the wave-offering of the "firstfruits" (Leviticus 23:10, 15) which, in turn seems to have been the Passover sabbath, on the fourteenth day of the first month (Exodus 12:2, 6). It was on the third day of the third month that God came down on Mount Sinai to give the law (Exodus 19:1, 11, 16). Jewish time-reckoning included both the first and last days of a time period in figuring the number of days between two events, so both the Lord’s appearance on Sinai and the annual Feast of Pentecost seem to have been 50 days after the Passover offering.
And so was the coming of the Holy Spirit! When the Holy Spirit came to the upper room, there were fiery tongues and a mighty wind (Acts 2:2-3). On that great day, Peter announced to Israel: "Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ" (Acts 2:36), and we have received His great promise of the indwelling Law in our hearts by the Holy Spirit (Hebrews 10:15-17). HMM