Mercedonius was eliminated by Julius Caesar when he introduced the Julian calendar in 45 BC.
[b] 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.
[c] However, the pontifex maximus would normally be an active politician, and the decision would often be manipulated to allow friends to stay in office longer or force enemies out early.
[13] However, following a discussion of intercalation by Michels (1967)[14] some specialist studies of the pre-Julian calendar published since 1967[15] claim that in intercalary years Februarius was set at either 23 or 24 days, which was followed by an intercalary month of 27 days.
The month was eliminated by Julius Caesar with his revised calendar in 45 BC.