The story of Passover is anti-science: a series of biological anomalies called “plagues” that are visited upon the Egyptians each time Pharaoh refuses to release the Israelite slaves; a sea which splits based on human appeal and divine command in order to allow those slaves, once freed, safe passage out of the land.

But that is not the point, it’s not meant to be science. The plagues which ravage the Egyptians are metaphors for the damage wroght by oppressive socieities, and the crossing of the Red Sea is a narrative metaphor for the spiritual and emotional journey of liberation.

Yesterday was Earth Day, and was a day for demonstrations supporting rooting public policy in scientific fact—the March for Science. Coming less than a week after the end of Passover, it is a reminder how Passover in its observance too is anti-science.

And that fact of anti-science has to do with precisely that, the end of Passover. For there are two traditions around when Passover ends, after seven days or after eight.

Here in America, we are more accustomed to eight days of Passover. To break down the difference: it is not so much that Passover here and elsewhere in the Diaspora is eight days long, it is that the chag, or festival day, is two days long. Passover in the Torah is described as lasting for seven days, with the first and the last days being full chagim, or festival days. The chagim have special observances (including the Seder on the first night), services, rituals and the prohibitions against work. The days in-between, days 2-6, are known as Chol Ha’Moed, they are part of the holiday, carry some of the observances of Passover, but do not have the same level of observance as the chagim.

In ancient times, the calendar would be set by the court in Jerusalem that would hear from witnesses who came to testify that they saw the new moon. Once their veracity was determined, the court would set the beginning of the month, and once it was certain when the new month began, the times of the holidays would become known. It was therefore important to be accurate, because the community would of course want to celebrate the holiday on the correct day.

It was therefore important that the court let the outlying communities in the Diaspora know that the new moon was set. However, with communications not being what they are now (think signal fires on hills and messengers, not tweets) it would sometimes take a while to get the word out (plus some annoying folks who would try to mess things up.) In the outlying communities it then became the practice to observe holidays for two days to be certain that they got the right day.

So for Passover, the first and the last days, which are chag, are doubled, so the first and the second and the seventh (and thus, now, the eighth) are considered chag, full festival days. Passover became an eight-day holiday because the last day of chag is doubled.

By the time of the Talmud, the community became more sophisticated and able to determine the calendar with certainty. However, the tradition of maintaining two-day festivals in the Diaspora was maintained based on the principle of minhag avoteichem, one keeps the custom of your ancestors.

Therefore an eight-day-long Passover is based on folk custom and lore rather than on science.

Getting back to the story, an eight-day Passover provides a spiritual and theological challenge as well. For it is on the seventh day that is traditionally seen as the anniversary of the crossing of the Red Sea. This “unscientific” climactic moment does, as mentioned, mark the final passage of the Israelites in their journey out of oppression, from the narrowness of Egypt/Mitzrayim to the vast expansiveness of the wilderness. It is fitting to end the holiday, which marks this journey, there. To extend the holiday is to extend the story to when the Israelites, having left Egypt, now find themselves adrift, angry, fearful, thirsty, nostalgic and resentful. Important to acknowledge, but for another time. We should end Passover on a theological “high note” of redemption, promise and hope.

Growing up I kept an eight-day Passover, as an adult I made the switch to seven, primarily for these reasons but also for Jewish unity between Israel and the Diaspora (not because I was craving pizza and beer earlier). I recognize that individuals can and should make their own choice about it.

But both because of science (the calendar) and anti-science (the story of the Red Sea), I believe we should keep a seven-day Passover.