The date for the Feast of Weeks must be counted beginning on the day the first of the firstfruits was offered (Lev. 23:14-16, Deut. 16:9-10).
The authorities in the Church of God are to count the days towards Pentecost, not “from,” but “beginning with” the starting point, as per the original Hebrew wording of Leviticus 23:15-16. The English word “from” is therefore misleading. The New American Bible (1970) makes the correct method of counting very clear: “Beginning with the day after the sabbath, the day on which you bring the wave-offering sheaf, you shall count seven full weeks, and then on the day after the seventh week, the fiftieth day,” you shall keep the feast of firstfruits (Lev. 23:15-16).
The day of the wave offering, the Sunday during the feast of Unleavened bread, was day one. Day seven would be the next weekly Sabbath. Day 49 would be the seventh Sabbath, and the 50th day would be a Sunday, the day after the seventh week, or seventh sabbath. This is how the original Hebrew and the authorized version state it. Thus, Pentecost always falls on a Sunday.