Undecimber or Undecember is a name for a thirteenth month in a calendar that normally has twelve months. Duodecimber or Duodecember is similarly a fourteenth month.
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Etymology
The word undecimber is based on the Latin word undecim meaning "eleven". It is formed in analogy with December, which, though the twelfth month in the Gregorian calendar, derives from decem meaning "ten". The word undecember (abbreviated Vnde) is recorded from a Roman inscription according to the Oxford Latin Dictionary, which defines it as "a humorous name given to the month following December".
Some recent authors report the names "Undecember" and "Duodecember" for the two intercalary months inserted between November and December upon the adoption of the Julian calendar in 44 BC, including the World Calendar Association and Isaac Asimov. This claim has no contemporary evidence; Cicero refers to the months as intercalaris prior and intercalaris posterior in his letters.
Historian Cassius Dio tells that Licinus, procurator of Gaul, added two months to the year 15 BC, because taxes were paid by the month. Though not named by Dio, who wrote in Greek, August Immanuel Bekker suggested these might have been called "Undecember" and "Duodecember". This may have been the origin of the claim.
Computing
In the Java Platform, Standard Edition, the java.util.Calendar class includes support for calendars which permit thirteen months. Although the Gregorian calendar used in most parts of the world includes only twelve months, there exist some lunar calendars that are divided into synodic months, with an intercalary or "leap" month added in some years. For example, in the Hebrew calendar seven years out of every nineteen (37%) have the "embolismic month" Adar II. The constant java.util.Calendar.UNDECIMBER represents such a month.