Q. Why is Pesach so late this year? Why does it sometimes coincide with Easter and sometimes not?
According to the Torah, Pesach must fall in the month of Aviv – the spring month – and the spring, like all the seasons, is governed by the sun.
As the Jewish calendar has roughly 354 and a quarter days, eleven less than the solar year, there is a corrective mechanism to ensure that Pesach will not end up too far away from the northern hemisphere spring.
This corrective mechanism is the extra month of Adar that comes seven times in every 19 years, ensuring that there will be a correlation between lunar and solar years.
The result is that in a leap year, when we have the extra Adar, Pesach is relatively later, and in a non-leap year it is relatively earlier.
In early church history there were certain communities who celebrated Easter on 15 Nisan, the full moon, when the Jews kept Pesach. In an attempt at separating the two events the first Council of Nicaea decided in 325 CE that Easter should always fall on a Sunday, which rarely coincided with the full moon.
Which Sunday? By the eighth century the rule was established that it should be the Sunday after the spring full moon.
However, the Christian and Jewish calculations of the new moon are not quite the same. Hence on some occasions Easter Sunday falls on the first day of Pesach, but it is rare to have a first day of Pesach on Sunday (with the accompanying problems of Erev Pesach being on Shabbat).