Passover came after Easter this year.

I will be going to a Seder tonight. Passover is late this year, and it is a rare year in that Passover comes after Easter. At the 1999 Drosophila Conference in Seattle the "fly board" first considered when to hold the 2005 Drosophila Conference. I thought I knew that Passover always follows Easter, so when someone (Elaine Strauss of the GSA) found that Easter was on March 26, I thought that we could avoid Passover by choosing later dates. I think that I even said so, so when someone else (Thom Kaufman) came up with a later date for Passover (April 24), I was surprised.

Theologically, it makes sense to celebrate Easter after Passover, because the Last Supper was a Passover Seder. Accordingly, early Christians did indeed celebrate Easter in "relationship with the full moon that falls during the Jewish month of Nisan" (from "Calendar," by David Duncan). However, the Christian world does not wish to tie their calendar to the Jewish calendar. Instead, Easter is usually celebrated on the first Sunday after the first full moon after the vernal equinox. Since Passover is almost always the first full moon after the vernal equinox, that amounts to the same thing. In fact, the Catholic church found it necessary to develop a lunar calendar for the purpose of dating Easter; a very useful discussion of this and other calendars can be found in the Catholic Encyclopedia. Another good book on this topic is "The Sun in the Church: Cathedrals as Solar Observatories" by J.L. Heilbron. In this century, Passover occurs no earlier than March 26 and no later than April 24, so this year is as late as it gets (the Naval Observatory provides a nice table of actual dates). I see that Adar II occurs in 2000, 2003, 2005, 2008, 2011, 2014, 2016, 2019, 2022, etc. (according to the 19 year Metonic cycle), with the latest dates for Passover occurring whenever this leap month occurs after two rather than three years. Passover falls after Easter in 2008, 2016 and 2024, three times in each 19 year cycle. There is a lot more to say about this, and I haven't even started on the Chinese calendar (more on that later).