Intercalans

…27 or 28 days, called Mercedonius, kept the calendar in step with the seasons. The confusion was compounded by political maneuvers. The Pontifex Maximus and the College of Pontiffs had the authority to alter the calendar, and they sometimes did so to reduce or extend the term of a particular…

…the seasons, an intercalary month, Intercalans, or Mercedonius (from merces, meaning wages, since workers were paid at this time of year), was inserted between February 23 and 24. It consisted of 27 or 28 days, added once every two years, and in historical times at least, the remaining five days…