Following the basic instructions about Pentecost's location on the calendar in Leviticus 23:10-16, we find that when Israel came into Canaan, they were to count beginning with the day following a Sabbath. Without further instruction, there could be a whole year's worth of Sabbaths to choose from! However, within Leviticus 23, the annual Sabbaths are arranged chronologically beginning with Nisan (also called Abib). This, combined with information obtained from other portions of the Bible, has led all concerned to conclude that the Sabbath in question is early in the year, located near the beginning of a spring harvest, and is one of three within the Days of Unleavened Bread. The church of God and the various sects of the Jews are in agreement on this.
The count is to continue fifty days with the fiftieth day being the Day of Pentecost. As a Greek word, the name Pentecost does not appear in the Old Testament, only in the New, and it means "fiftieth." In the Old Testament, Pentecost is called "the Feast of Weeks" or "the Feast of Firstfruits."
Carefully note that God points only to a Sabbath—it must first be found—in order to begin the count. This fits nicely within God's directive in Exodus 31:13 that the Sabbath is a sign between Him and His people. Day One of the count does not begin with a Sabbath, but with the day following it. However, without first isolating which Sabbath, one cannot know which "morrow"—which day after. If one does not use the correct Sabbath, it may set Pentecost's observance as much as seven days off God's intended target.
The Sabbath in question here can be neither the First nor the Last Day of Unleavened Bread, though both are annual Sabbaths. Why? Because using either of those holy days, both of which fall on fixed dates, effectively eliminates a person's need to count! This is because, when one begins counting fifty days from a fixed date, one will always end on a fixed date.
If we begin to count with the day following Nisan 15 (the First Day of Unleavened Bread), we will always end on Sivan 6. If we commence our count on the day following Nisan 21 (the Last Day of Unleavened Bread), we will always finish on Sivan 12. If God wanted us to observe Pentecost on a fixed date, He would have told us so, even as He did with all the other festival dates in Leviticus 23.
One man suggested that counting from a fixed date is still counting. Yes, that is true. But if one does that, the count only has to be done once in all of history, and Pentecost's location is found forever. The man's suggestion is similar to interpreting that the command to eat unleavened bread during the Days of Unleavened Bread no longer applies because the Israelites did it when they first came out of Egypt! Even as unleavened bread must be eaten each year, the clear implication from Leviticus 23 is that God wants us to count to Pentecost afresh each year.
God wants us to count to Pentecost year-by-year beginning with the day following a Sabbath whose date changes from year to year. This can only be the weekly Sabbath that falls on or between the two holy days during the Days of Unleavened Bread. The starting point has been located. Even though the count does not actually begin with the Sabbath, the Sabbath's location is of primary importance, not the day after. The day after would never be located without first locating the correct Sabbath.