Mercedonius, also known as Intercalaris, was the intercalary month added in leap years of the Roman calendar. The resulting year was either 377 or 378 days long. The exact mechanism by which this was done isn't clearly specified in ancient sources. It is generally agreed by modern scholars that Februarius was truncated to 23 or 24 days, to be followed by an intercalary month of 27 days. As a result, the year was lengthened by a total of 22 or 23 days. Romans believed that the month had been adde… (More on Mercedonius)