New Year’s Eve and New Year’s Day together constitute a very strange holiday: Mardi Gras, Lent, and Easter all crammed into one stretch of about 12 hours, from around 8 p.m. on December 31 to around 8 a.m. on January 1, depending on how hard you like to party, with the Lenten part extending as long as your resolutions do.

Whereas Thanksgiving is a Christian holiday disguised as a secular one, New Year’s remains true to its pagan roots—alongside Halloween, which has turned into something of a heathen cultural juggernaut, it is our most obviously pagan festival. We owe the date itself to Julius Caesar’s calendar reforms, which moved the date of the new year from the Ides of March to January 1 and the patron of the celebration from the war god Mars to the two-faced Janus, the much more ancient personification of beginnings and endings. Knowing the calendar adds a little richness to the story of Caesar’s assassination on the Ides of March, the most political day of the Roman year, the day on which new consuls began their terms, with the Romans counting their years from one consulship to the next in roughly the same way later Europeans counted their years from the coronations of their kings. Assassinating Caesar on the Ides of March was profoundly conservative in its symbolism: Caesar’s tyranny was of a piece with his contempt for Roman mores and tradition.