Pentecost is one of the Feasts of the LORD and in Hebrew it is called Shavuot. The English word Pentecost comes from the Greek Word pentekoste and it means the 50th day. In Leviticus 23, there is a prescription that states you shall count 50 days to the day after the seventh sabbath. The date of Pentecost is determined by which date the feast of First Fruits falls on.

There are two basic views on what date the Feast of First Fruits is on and the reason is because Leviticus 23:11 says that this is celebrated the day after the Sabbath. Since there are two types of Sabbaths in this scenario (The special Feast Sabbath and the weekly Sabbath.), it is not completely clear which Sabbath is being referred to.

Leviticus 23:11 and he shall wave the sheaf before the LORD, so that you may be accepted. On the day after the Sabbath the priest shall wave it.

1st View: The special Sabbath, which is the first and last day of Unleavened Bread. This view causes Pentecost to be on a set day that is determined by the Hebrew Month Nissan 15 because that’s when Unleavened Bread starts. 

2nd View: The weekly Sabbath always causes Pentecost to be on a Sunday since Sunday would always be the day after the weekly Shabbat.

Personally, I hold the 2nd view since there is no specific Hebrew date given like it does for other feast days. However, in my experience, most Messianic Jews hold to the first view.