Today is Tuesday. Sundown tonight is 'tomorrow' on Jewish calendars. Tonight, as in many years, the Feast of Weeks (Pentecost) is being celebrated mid-week. This fact reflects the victory of Pharisaic influence on Judaism in the centuries after Jerusalem's destruction.
Before 70 AD, the Pharisees used to insist that Shavuot be observed on Sivan the 6th, precisely fifty days from the Sabbath of Passover's first day. The Sadducees, however, favored an interpretation of Mosaic Law which began counting from the Saturday during Passover. This, of course, always left Shavuot on a Sunday, which pleased the Sadducees and their wealthy constituency.
Just think about travelers and money. Pentecost is a one-day festival, but also a major one, drawing multitudes of pilgrims to the Holy city every year. While the Sadducees ruled, the Feast always began at sundown on Saturday, so pilgrims had to arrive by Friday and could not depart until Monday. That meant more money being spent in town, which - again - was very good news for the wealthy ruling class of Jerusalem, not to mention the Sanhedrin itself. Decisions like this were undoubtedly what kept the Sadducees in power for so long.
We close with a side note, perhaps of more interest to most of you:
It is most likely (*) that Sivan 6th fell on a Saturday/Sunday in the year 33 AD. That meant both Jerusalem AND God agreed on when Pentecost should be held that particular year. This may not be a conclusive point, historically, but it does add some weight for me personally in confirming that 33 AD was the Pentecost of Acts 2, seven weeks to the day after Resurrection Sunday.
(*) These dates hold only IF the lunar observation was on target that year, which we cannot strictly assume, but which should have been true at least more often than not. For more about lunar considerations, see Dating the Crucifixion: Possible Friday Passovers, and Dating the Crucifixion: Sadducees, Calendars and Festival Finances, and works referenced in those posts.