A Calendar the Priests Could Bend

Before Julius Caesar's reform, the Roman calendar was lunar in origin and needed an intercalary month inserted periodically to keep pace with the solar year, and the decision of when and whether to intercalate belonged to the pontifices, the priestly college responsible for the calendar's upkeep. That discretion was abused for ordinary political advantage: lengthening a friendly magistrate's term or shortening a rival's simply by deciding whether the current year would run long, and by the first century BCE the civil calendar had drifted badly out of alignment with the actual seasons it was meant to track.

Caesar's reform of 46 BCE, carried out with the astronomer Sosigenes, fixed the year at 365 days with a leap day added every fourth year, removing the discretionary intercalation that had made the calendar a political instrument and giving the fasti a stable solar frame that has run, with the later Gregorian correction to its leap rule, into the calendar most of the world uses today. The Julian Calendar entry in this atlas covers that civil reform itself; the fasti covers the religious classification of days and the festival cycle that reform stabilised rather than replaced.