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. Unfortunately the pontifex maximus, who would normally be an active politician, often manipulated the decision to allow friends to stay in office longer or force enemies out early. Such unpredictable intercalation meant that dates following the month of Februarius could not be known in advance, and further to this, Roman citizens living outside Rome would often not know the current date.
Source: Mercedonius – Wikipedia
See also: Undecimber – Wikipedia