Leviticus 23:10-15

23:10 “Speak to the Israelites and tell them, ‘When you enter the land that I am about to give to you and you gather in its harvest, then you must bring the sheaf of the first portion of your harvest to the priest, 23:11 and he must wave the sheaf before the Lord to be accepted for your benefit – on the day after the Sabbath the priest is to wave it. 23:12 On the day you wave the sheaf you must also offer a flawless yearling lamb for a burnt offering to the Lord, 23:13 along with its grain offering, two tenths of an ephah of choice wheat flour mixed with olive oil, as a gift to the Lord, a soothing aroma, and its drink offering, one fourth of a hin of wine. 23:14 You must not eat bread, roasted grain, or fresh grain until this very day, until you bring the offering of your God. This is a perpetual statute throughout your generations in all the places where you live.

The Festival of Weeks

23:15 “‘You must count for yourselves seven weeks from the day after the Sabbath, from the day you bring the wave offering sheaf; they must be complete weeks.