The Ancient Math That Sets the Date of Easter and Passover

Let’s get some things straight.

Passover is a springtime Jewish festival celebrating the early Israelites’ exodus from Egypt and freedom from slavery. Jews observe it by hosting a ritual dinner, called a seder, and then by abstaining from eating all leavened bread for about a week. (Some of us abstain from some other stuff, too.) Instead, we eat matzo, a thin, unleavened cracker.

Easter is a springtime Christian holiday celebrating the resurrection of Jesus Christ and freedom from sin and death. It is preceded by a series of holidays commemorating Jesus’s path to the cross. One of these holidays is Maundy Thursday, which, aside from being a great name for a holiday, is a remembrance of the Last Supper, which was a seder. In the United States, many Christians observe Easter by