Here is T. W. Tramm's item for today May 2 2021 at Facebook where he explains all that he found about how this year Passover was celebrated before the spring equinox this year, about March 27.
The Bible indicates the first month, Nisan, starts on the new moon after the vernal equinox. Firstfruits (Resurrection) is the third day after Passover.
Tramm has written a new book, "Summer is Near," about 100 pages, and is studying this very closely. He tells so much in the posts at Facebook. Pentecost will actually fall June 20, the first day of summer. In 1948, Pentecost also fell on June 20, which was also the first day of summer so that is exactly to the day 73 years to this year. The Bible sets forth how Pentecost is always on "the day after the Sabbath" after a 49 day count (also from a Sunday), but the Jews now celebrate Pentecost on a fixed date every year.