Undecimber or Undecember is a name for a thirteenth month in a calendar that normally has twelve months. Duodecember is similarly a fourteenth month.
The word Undecimber is based on the Latin word undecim meaning "eleven". This is intended by analogy with December, which, though now the twelfth month, derives from decem meaning "ten". The "i" in Undecimber is therefore correct, even though December is spelled with an "e". The word Undecember (abbreviated as Vnde) is recorded from a Roman inscription, apparently as "A humorous name given to the month following December".
When the reformed Julian calendar was introduced in 44 BC, the discrepancies accrued to that point were rectified by inserting two intercalary months, totalling 67 days, between November and December. Some recent authors report the names "Undecember" and "Duodecember" for these, including the World Calendar Association and Isaac Asimov. This claim has no contemporary evidence; Cicero's letters of the time refer to the months as intercalaris prior and intercalaris posterior.
Historian Cassius Dio tells that Licinus, procurator of Gaul, added two months to the year a.u.738 (15 BC), because taxes were paid by the month. Though not named by Dio, who wrote in Greek, August Immanuel Bekker suggested these would have been called "Undecember" and "Duodecember".Glare, P.G. (2002). Oxford Latin Dictionary. Clarendon Press. ISBN 0-19-864224-5. Jézéquel, Jules (1937). "Why the World Needs This Reform". Journal of calendar reform (New York City: World Calendar Association) : 64. Asimov, Isaac; John Bradford (1963). The clock we live on (revised ed.). Collier Books. p. 118. ISBN 0-200-71100-8. Heitland, W.E. (1909). "Chap LVIII: From the Battle of Thapsus to the death of Caesar: 46–44 B.C.". The Roman Republic. Vol.3. Cambridge: Cambridge University Press. pp. 347 §1269. ISBN 0-89005-577-7. Cassius Dio (1914-27). "LIV 21.5". Roman History. Loeb Classical Library. translated by Earnest Cary. Harvard University Press. p. 335. ISBN 0-665-72855-7. Check date values in: |date= (help)
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.Janert, Philipp K. (2007-06-04). "Making Sense of Java's Dates". On Java. O'Reilly Media. Retrieved 2007-12-13. "java.util Class Calendar: UNDECIMBER". Java Platform, Standard Edition 6: API Specification (Sun Microsystems). 2008. Retrieved 2009-05-22.