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Although there is an older history from the reign of Constantine in the 4th century, the modern duel between December 25 and January 6 dates from 1582, when Pope Gregory XIII instituted a new calendar to correct the ongoing drift between a 365-day calendar and a slightly longer astronomical year. The non-integral mismatch between earthly days and earthly years meant that, with each passing calendrical year, dates diverged a little further from the equinoxes. The Gregorian solution initiated our practice of "leap years," whereby one day is added to the calendar every four years. (The reality is in fact slightly more complicated. Since the earth takes 365.2422 days to orbit the sun, rather than 365.25, leap years are defined as years divisible by 4, not including most years divisible by 100, but including years divisible by 400.)

Some Catholic countries adopted the new calendar immediately, but other countries came on board decades or centuries later, having drifted yet further from their appropriate solar positions. In Britain, legislation called the Calendar Act was passed in 1751, by which time the drift was approximately twelve days, so twelve days were dropped from the old Julian calendar. The result was that in 1752 Christmas was celebrated on December 25, but purists clung to the old date of January 6. For readers interested in what Ithaca calls the "parallactic drift of socalled fixed stars, in reality evermoving wanderers," the inexactitude of human calendars provides an additional, temporal way to contemplate the concept of parallax.