Michael Colunga (10 May 2011)
"What determines the start of the month of Nisan, and therefore Passover, Pentecost, etc."

Hello, John and Doves,
Here is a great site to get acquainted with all kinds of aspects of the Jewish calendar:
You may disagree with these findings, but I believe you would be wise to seriously consider these claims.
Baruch HaShem,
Mike C.