Does Pentecost begin the evening before May 26 or the evening before May 30?
 
Here is what YHWH commanded Israel:
 
Lev. 23:15,16   "You shall also count for yourselves from the day after the sabbath from the day when you brought in the sheaf of the wave offering, there shall be seven complete sabbaths.   You shall count fifty days to the day after the seventh sabbath; then you shall present a new grain offering to the LORD."
 
This is very precise and it sounds simple enough.  However, the problem is that besides the weekly sabbath there are seven other sabbaths in the year.  These are the high sabbaths and one of them is on the first day of Hag HaMatzah (Feast of Unleavened  Bread) which occurs on Nisan 15.
 
So, were they to count the 7 weeks from the day after the weekly sabbath or the day after Nisan 15?  This caused a big controversy between the Saducees and the Pharisees.
 
The Saducees said to count 50 days from the weekly sabbath that came after Passover and during the week of unleavened bread.  This way Shavuot (Pentecost) would always be on a Sunday.
 
The Pharisees said to count 50 days from Nisan 15.  This way Shavuot (Pentecost) would always be on Sivan 6.
 
Mainstream Judaism and some Messianic Jews today follow the teaching of the Pharisees and always celebrate Shavuot on Sivan 6   (actually is starts the evening before).  So this year it begins Tuesday evening,  May 25 and goes through Wednesday, May 26.
 
Some say that the only way you can count 50 days to the day after the seventh sabbath (as it says in Lev.23:16) is to follow the counting method of the Saducees and count from the weekly Sabbath that follows Passover.  Some Messianic Jews and others follow this practice so they will be celebrating Pentecost beginning Saturday evening May 29 and through Sunday May 30.
 
Shalom,  Roberta.