(EXPLAINER) Easter and Passover sometimes overlap because they are both based on lunar calendars — albeit different ones.

Since 2000, the two festivities have overlapped every year but four — in 2005, 2008, 2016 and this spring.

Easter is calculated based on the Christian liturgical calendar tied to the first Sunday following the first full moon after the vernal equinox. Passover, on the other hand, is determined by the Jewish calendar, which is also lunar-based.

That’s where the similarities end.

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In fact, they don't always overlap due to differences in the specific rules and calculations used in each tradition. For example, while Easter always falls on a Sunday, Passover can begin on any day of the week.

In addition, variations in the length of lunar months and different methods of intercalation (adding extra months to synchronize with solar years) contribute to the occasional misalignment of the two holidays.

Here are other areas where Easter and Passover overlap:

The Last Supper

The Last Supper, as described in the New Testament, is traditionally understood to have been a Passover seder.

According to the Gospel accounts, Jesus and his disciples gathered to celebrate the Passover together in Jerusalem. During this meal, Jesus instituted the sacrament of Communion or the Eucharist, using bread and wine to symbolize his body and blood.

The Gospel of Luke explicitly mentions that the Last Supper took place during the time of the Passover feast (Luke 22:7-8).

However, there is some scholarly debate about the exact timing and nature of the meal, with some interpretations suggesting it may have occurred on the day before Passover or during the Passover preparations. Nonetheless, the traditional understanding within Christianity is that the Last Supper was indeed a Passover meal.

Freedom from bondage

Indeed, both Easter and Passover have themes of liberation central to their celebrations.

Passover commemorates the freedom of the Israelites from slavery in ancient Egypt, as recounted in the Hebrew Bible (Exodus 12-13). The Passover story revolves around the 10 plagues sent by God to persuade the pharaoh to release the Israelites — culminating in the Passover event itself, where the Israelites were spared from the final plague, the death of the firstborn, by marking their doorposts with the blood of a sacrificial lamb.

Similarly, Easter for Christians celebrates the resurrection of Jesus Christ, which is understood as liberation from sin and death. Christians believe that through Jesus' death and resurrection, believers are freed from the power of sin and offered eternal life. The resurrection is seen as a victory over sin and death, offering hope and liberation to humanity.

Both holidays, in fact, share the theme of liberation, although they commemorate different historical events and have distinct theological interpretations.

Beyond celebrating a single day

Both Easter and Passover are observed over a span of time, encompassing various rituals, observances and traditions beyond a single day.

Passover is an eight-day festival (seven days in Israel) that commemorates the Exodus story and the Israelites' liberation from slavery in Egypt. The first two days (or the first and last two days in Israel) and the seventh day (eighth day in Israel) are typically considered major holidays with special prayers and rituals.

Throughout the entire festival, Jews engage in various practices such as removing chametz (leavened bread) from their homes, conducting a Seder meal on the first two nights, and refraining from consuming chametz for the duration of Passover.

Easter, meanwhile, is preceded by the season of Lent, which lasts for 40 days (excluding Sundays) and is a time of fasting, prayer and penance in preparation for Easter Sunday. The week leading up to Easter Sunday is called Holy Week and includes significant events such as Palm Sunday (commemorating Jesus' triumphal entry into Jerusalem), Maundy Thursday (commemorating the Last Supper), Good Friday (commemorating the day Jesus was crucified) and Holy Saturday.

Easter Sunday marks the culmination of Holy Week and the celebration of Jesus’ resurrection.

Clemente Lisi is the executive editor of Religion Unplugged. He previously served as deputy head of news at the New York Daily News and a longtime reporter at The New York Post. Follow him on X @ClementeLisi.