Easter Sunday has just passed. In some ways, the celebration is a strange one. Why does the holiest of Christian holidays bounce around the calendar? Why do small children search for eggs and worship a large rodent?

In fact, you can blame those Easter oddities on the moon. The date of Easter harkens back to the Jewish Passover (or, more correctly, Pesach) celebration. Both are celebrations of spring, when the world is reborn in a flurry of new life. They are both connected with the moon, which grows and then shrinks over the course of about a month. For one day each month, the moon disappears and then is reborn as a thin crescent.

The death and rebirth of the moon symbolizes in cultures around the world the death and rebirth that are necessary if we and the planet we live on are to be renewed.

The Jewish calendar is based on the moon and not the sun as our modern Gregorian calendar is. The number of lunar cycles in a year does not divide evenly into the solar year, so the Passover celebration begins on a different day each year. Generally speaking, Passover began on the evening of the first full moon after the vernal equinox, the first day of spring.

During the early days of Christianity, no one knew exactly when to celebrate Easter. To settle the matter, Constantine, the Christian Emperor of Rome, convened the Council of Nicaea in 325 A.D. The council decreed that Easter would happen every year on the Sunday following the first full moon after the vernal equinox. If the full moon happened on a Sunday, then Easter would be celebrated on the following Sunday. The council also ruled that Easter had to occur after the beginning of Passover.

Roman Catholic and Protestant churches eventually abandoned the Passover rule. The Eastern churches did not, which is why the Eastern Orthodox Church sometimes doesn’t celebrate Easter on the same Sunday as other Christian churches.

If all of this seems complicated, note that I have oversimplified it, as members of the Jewish and Eastern-Orthodox communities will probably notice. The Eastern Orthodox Church uses the Roman Julian calendar, not our modern Gregorian calendar, to determine these dates, so sometimes the two Easters don’t occur of the same date for that reason, as well.

This year, the intricacies of the Jewish lunar calendar determined that Purim, not Pesach, began on March 23. Pesach begins on the next full moon evening, April 22.

Our calendar difficulties all arise because of fundamental astronomical mismatches among our methods of determining time. The lunar cycle (i.e., the month) does not divide evenly into the year, the orbit of Earth around the sun. Heavens, the length of a day doesn’t divide evenly into either the month or the year. We must do the best we can with what God and/or nature gave us.

The moon also figures into the Anglo-American habit of celebrating Easter through the distribution of eggs to children.

When the early Catholic Church began to spread its religious beliefs in England, they realized that they could make their religion more palatable by incorporating some of the practices of the indigenous religions. In those days, the English worshiped many gods. Among them was Ostara – Eostre to the Romans – the goddess of springtime, who made the warn weather come and the world burst forth with new life.

Near the first day of spring, when her powers were at their greatest, Eostre gathered little children around her and performed feats of magic. One fine spring day, as the children sat before her, a beautiful songbird landed on her outstretched finger. To the delight of the children, she transformed the bird into a rabbit. The children squealed with delight until they saw that the rabbit was weeping. It had been a bird that could fly high above the clouds, safe from all harm. Now it was a rabbit, prey to all the animals that love to eat bunnies for breakfast.

The children begged Eostre to change the rabbit back into a bird, but this she could not do. Instead, on the first day of spring, when her powers are at their greatest, she can transform bunny back to bird for a single day. On that day, the bird lays her eggs.

The next day, bird must become bunny again. However, she remembers the children who took pity on her. On that day, she delivers her eggs to all the children of the world. On all other days, the rabbit lives on the moon, where she may still soar high above the clouds, safe from harm. If you look at the dark markings on the full moon, you can see the outline of the rabbit to this very day.

And that is why, on the highest day of the Christian calendar, the Eostre Bunny still delivers her eggs to the descendants of the children who took pity on her so long ago.

The egg symbolism associated with early spring has spawned some pretty strange beliefs and practices that continue to this day. More on that next week.