Mercedonius
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Mercedonius, also known as Intercalaris, was the intercalary month (added in leap years) of the Roman calendar. It had 27 days, beginning after the 23rd or 24th day of Februarius, so that the year was lengthened by a total of 22 days. Romans believed that the month had been added to the Roman calendar (along with Januarius and Februarius) by King Numa Pompilius in the 7th century BC.
This month was supposed to be inserted every two years to realign Numa's 355 day lunar calendar with the solar seasons (lunisolar calendar). The average year thus created was 366 1/4 days (over one day longer than the tropical year) and the pontifices often forgot to insert the month at the proper time or deliberately inserted it early or late to allow some officials to stay in office longer or force others out early.
The month was eliminated by Julius Caesar when he introduced the Julian calendar in 46 BC. The name Mercedonius comes from merces meaning "wages" as workers were paid at that time of year.
