Leviticus 23:15,16

From the morrow From the sixteenth day of the month, and the second day of the feast of unleavened bread, inclusively; seven sabbaths shall be complete Namely, forty-nine days; unto the morrow after the seventh sabbath Which made just fifty days; whence this feast, from a Greek word, πεντηκοστη, pentecoste, which signifies the fifteenth day, was called pentecost. Ye shall offer a new meat (or flower) offering Another first-fruit-offering, made of wheat, which was then ripe.