In addition to the familiar seven-day week, the Scriptures also introduce us to the idea of a week of weeks, a period of forty-nine days:
Feast of Weeks/Pentecost – the firstfruits of wheat
Celebrate the Feast of Weeks with the firstfruits of the wheat harvest.
From the day after the Sabbath, the day you brought the sheaf of the wave offering, count off seven full weeks. Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to the LORD.
On that same day you are to proclaim a sacred assembly and do no regular work.
On the day of firstfruits, when you present to the LORD an offering of new grain during the Feast of Weeks, hold a sacred assembly and do no regular work.
Count off seven weeks from the time you begin to put the sickle to the standing grain. Then celebrate the Feast of Weeks to the LORD your God by giving a freewill offering in proportion to the blessings the LORD your God has given you.
When does this 49-day count begin? On the “day after the Sabbath, the day you brought the sheaf of the wave offering.” This is the wave offering of the firstfruits of the barley harvest, which happens soon after Passover:
Passover and wave offering – the firstfruits of barley
The LORD’s Passover begins at twilight on the fourteenth day of the first month. On the fifteenth day of that month the LORD’s Feast of Unleavened Bread begins; for seven days you must eat bread made without yeast. On the first day hold a sacred assembly and do no regular work. For seven days present an offering made to the LORD by fire. And on the seventh day hold a sacred assembly and do no regular work.
When you enter the land I am going to give you and you reap its harvest, bring to the priest a sheaf of the first grain you harvest. He is to wave the sheaf before the LORD so it will be accepted on your behalf; the priest is to wave it on the day after the Sabbath.
Passover begins on the evening of month 1, day 14. This is somewhat ambiguous, having sometimes been interpreted as the evening on which day 14 ends, and sometimes as the evening on which day 14 begins, but I believe the second interpretation to be the better one. So day 14 of the month is Passover, then day 15 of the month is day 1 of the feast of unleavened bread (sometimes also referred to as Passover day 1). This day is set aside as a day of “no regular work,” a Sabbath. Then day 16 of the month is the “day after this Sabbath.” For the wave offering, again there are two interpretations that have been followed. One is that the wave offering is on day 16, the day after this Sabbath. I believe this is most commonly followed. The other, the Karaite interpretation, is that the wave offering is the day after the next weekly Sabbath. This wave offering is then day one of the count of 49 days, also known as the “Omer count,” followed by Pentecost on day 50.