The timing of a Sunday Pentecost, is different from the timing of a Rabbinic Pentecost. This is because the count of fifty days always begins the first Sunday after the First Day of Unleavened Bread.
Method of Calculation
The reckoning of the date of Pentecost is only found in one place in the Bible, which is Leviticus 23:15-16 :
- And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete:
- Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD.
The "sabbath" referred to in verse 25 is interpreted in this case as the weekly Sabbath. Therefore the "morrow after the sabbath" is always a Sunday, and Pentecost will then always occur on a Sunday.