Many might not know that the date of the Easter holiday each year has some surprisingly lunar connection: It is always the first Sunday after the first full moon after the Spring Equinox, which was earlier this week, “pink moon.”

The Spring Equinox is always on March 20, which, according to the moon’s rotation around Earth, does not always mean the date will match up with the lunar calendar. In 2016, March 20 fell on a Sunday and the full moon came shortly thereafter, making Easter squarely on March 27th. But this year, the equinox was on a Monday and the full moon was a week prior. So, we had to wait almost the entire lunar cycle until April 12 for the full moon. Now, this Sunday, Christians can finally celebrate Easter.

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Easter was designated a “moveable feast” by the First Council of Nicaea, or council of Christian churches, in the year 325. Early Christians wanted to ensure that Easter would always be during a full moon so that believers could make their pilgrimage to Jerusalem to honor the day he rose. It’s hard enough to fathom what 325 years after Christ was like, let alone 2,017 years after and for that reason, there really is no date of Christ’s resurrection set in stone.

This all got even more murky when scientists decided to change the set date for the spring equinox from March 21 to March 20th. So, even that date is not set in stone. Unfortunately, we can’t even count on the moon entirely either. It orbits around earth about every 27 days and seven hours but, the Earth technically orbits the sun every 364.5 days, so scientists created the Leap Year to keep our timing on Earth in sync with the Sun. So, every four years, the lunar calendar shifts, as well.

Ultimately, we are depending on three very irregular intervals of time to celebrate an event in which no one even has a date for. Nonetheless, Christian people across the globe will be celebrating their faith with feasts, church masses, and Easter egg hunts.

Photos via Flickr / cornishlens.co.uk