It is formed in analogy with December, which, though the twelfth month in the Gregorian calendar, derives from decem meaning "ten".
[2][1] Some recent authors have reported 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[3] and Isaac Asimov.
[6] Though not named by Dio, who wrote in Greek, August Immanuel Bekker suggested these might have been called "Undecember" and "Duodecember".
[7] In the Java Platform, Standard Edition, the java.util.Calendar class includes support for calendars which permit thirteen months.
For example, an organization may wish to adjust its books to reflect the fact that some of its sales and resulting payments due from customers will not be paid.