Susan Stolovy, an astronomer at Steward Observatory (Tucson, USA), has been kind enough to calculate the date of Passover for me for a wide range of dates around the time of the Nativity. 

Susan comments: "I'm getting my information from http://www.uwm.edu/cgi-bin/corre/calendar

The first day of Passover is always the 15th of Nisan (starting the evening of the 14th of Nisan).

Here are the Gregorian dates for Nisan 15. Note that the Jewish calendar has a 19-year cycle, in which the years 0, 3, 6, 8, 11, 14, and 17 all have an extra month of Adar (30 days) which occurs in Feb or March. I happened to notice that the 0th year of one of these cycles is the year 02 (which is 3762), so you can see why some years Passover is considerably later than others."

You can compare these dates with the dates of lunar eclipses visible from Jerusalem around this time. We know that Herod died between a lunar eclipse observed from Jericho and the date of Passover. In other words, to date Herod’s death, we are looking for an eclipse that happened less than a lunar month (29 days) before the date of Passover.

Only two eclipses in this interval fall around the time of Passover and, of these, the March 23^rd, 5BC eclipse was just after Passover. In other words, only the March 13^th 4 BC eclipse fits the known circumstances of Herod’s death. Curiously, it was the least spectacular of all the eclipses of the period in question.