Leviticus 23:16

Even unto the morrow after the seventh sabbath
Or weeks, forty nine days being counted, the following was the fiftieth day, or Pentecost:

shall ye number fifty days;
from whence this feast had the name of Pentecost, ( Acts 2:1 ) ; all in Israel were obliged to number those days, except women and servants ^F20: the manner of doing it was this ^F21; on the night of the second (day of the passover), after the evening prayer, they began to number; but if anyone forgot to number at the beginning of the night, he went and numbered all the night; for the commandment is for everyone to number by himself, and he ought to number standing, and to bless first, and number the days and weeks: How? on the first day he says, This is one day, until he comes to seven days, and then he says, This is the seventh day, which is one week; and on the eighth day he says, This is the eighth day, which is one week and one day, and so till he comes to the fourteenth; then he says, This is the fourteenth day, which make two weeks; and in this way he numbers, and goes on until the forty ninth day: and ye shall offer a new meat offering unto the Lord; that is, of new corn, as the Targum of Jonathan and Jarchi explain it, and this was of wheat; for it was the offering for the wheat harvest, which was offered on the fiftieth day from the offering of the sheaf or omer of the barley harvest.