Undecimber or Undecember is a name for a thirteenth month in a calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...

 that normally has twelve months. Duodecember is similarly a fourteenth month.


The word Undecimber is based on the Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 word undecim meaning "eleven". This is intended by analogy with December
December is the 12th and last month of the year in the Julian and Gregorian Calendars and one of seven months with the length of 31 days.December starts on the same day as September every year and ends on the same day as April every year.-Etymology:...

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
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

 was introduced in 44 BC, the discrepancies accrued to that point were rectified by inserting two intercalary
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

 months, totalling 67 days, between November and December. Some recent authors report the names "Undecember" and "Duodecember" for these, including the World Calendar
World calendar
The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930.-Features:The World Calendar is a 12-month, perennial calendar with equal quarters. It is perennial, or perpetual, because it remains the same every year.Each quarter begins...

 Association and Isaac Asimov
Isaac Asimov
Isaac Asimov was an American author and professor of biochemistry at Boston University, best known for his works of science fiction and for his popular science books. Asimov was one of the most prolific writers of all time, having written or edited more than 500 books and an estimated 90,000...

. This claim has no contemporary evidence; Cicero
Marcus Tullius Cicero , was a Roman philosopher, statesman, lawyer, political theorist, and Roman constitutionalist. He came from a wealthy municipal family of the equestrian order, and is widely considered one of Rome's greatest orators and prose stylists.He introduced the Romans to the chief...

's letters of the time refer to the months as intercalaris prior and intercalaris posterior.

Historian Cassius Dio tells that Licinus, procurator
Procurator (Roman fiscal)
A Roman fiscal procurator was the chief financial officer of a province of the Roman Empire during the Principate era...

 of Gaul
Gaul was a region of Western Europe during the Iron Age and Roman era, encompassing present day France, Luxembourg and Belgium, most of Switzerland, the western part of Northern Italy, as well as the parts of the Netherlands and Germany on the left bank of the Rhine. The Gauls were the speakers of...

, 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
Attic Greek
Attic Greek is the prestige dialect of Ancient Greek that was spoken in Attica, which includes Athens. Of the ancient dialects, it is the most similar to later Greek, and is the standard form of the language studied in courses of "Ancient Greek". It is sometimes included in Ionic.- Origin and range...

, August Immanuel Bekker
August Immanuel Bekker
August Immanuel Bekker was a German philologist and critic.-Biography:Born in Berlin, Bekker completed his classical education at the University of Halle under Friedrich August Wolf, who considered him as his most promising pupil. In 1810 he was appointed professor of philosophy in the University...

 suggested these would have been called "Undecember" and "Duodecember".


In the Java Platform, Standard Edition
Java Platform, Standard Edition
Java Platform, Standard Edition or Java SE is a widely used platform for programming in the Java language. It is the Java Platform used to deploy portable applications for general use...

, the java.util.Calendar class
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...

 includes support for calendars which permit thirteen months. Although the Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

 used in most parts of the world includes only twelve months, there exist some lunar calendar
Lunar calendar
A lunar calendar is a calendar that is based on cycles of the lunar phase. A common purely lunar calendar is the Islamic calendar or Hijri calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not linked with the seasons and drift each solar year by 11 to...

s that are divided into synodic months, with an intercalary
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

 or "leap" month added in some years. For example, in the Hebrew calendar
Hebrew calendar
The Hebrew calendar , or Jewish calendar, is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits , and daily Psalm reading, among many ceremonial uses...

 seven years out of every nineteen (37%) have the "embolismic month" Adar II. The constant
Constant (programming)
In computer programming, a constant is an identifier whose associated value cannot typically be altered by the program during its execution...

java.util.Calendar.UNDECIMBER represents such a month.
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