Leviticus 23:15

Sabbath. Not the ninth day of the week, but the first day of the Passover; from the morrow of which seven weeks or 49 days were reckoned; and the next day was Pentecost. (Menochius) --- They began, therefore, to count on the 16th of Nisan, and end on the 6th of the third month Sivan. All the intermediate days took their denomination from this second day of the Passover; so that the next Saturday was called the first sabbath after the second day; in Greek Deuteroproton, the second-first; (Luke vi. 1,) a term which had puzzled all the interpreters until Jos. Scaliger made this discovery. (Emend. 6.) The Samaritans count from the day after that sabbath which follows the Passover; so that if the festival fall on Monday, they celebrate Pentecost later than the Jews. See their Letter to Huntington. (Calmet)