Mercedonius (Latin for "Work Month"), also known as Mercedinus, Interkalaris or Intercalaris (), was the intercalary month of the Roman calendar. The resulting leap year was either 377 or 378 days long. It theoretically occurred every two (or occasionally three) years, but was sometimes avoided or employed by the Roman pontiffs for political reasons regardless of the state of the solar year. Mercedonius was eliminated by Julius Caesar when he introduced the Julian calendar in 45 BC.
History
This month, instituted according to Roman tradition by Numa Pompilius, was supposed to be inserted every two or three years to align the conventional 355-day Roman year with the solar year.
The decision of whether to insert the intercalary month was made by the pontifex maximus, supposedly based on observations to ensure the best possible correspondence with the seasons.