Mercedonius is an ancient Roman month that was added to the calendar to help align it with the lunar year. It was typically inserted between February and March, making the Roman calendar more accurate. This month was not fixed and could vary in length, often lasting 22 or 23 days. The introduction of Mercedonius was part of the efforts by the Romans to synchronize their calendar with the seasons. The Roman calendar underwent several reforms, including the more well-known Julian calendar introduced by Julius Caesar in 45 BCE, which eliminated the need for Mercedonius.