Undecimber Definition
    pronoun
  
 A fictitious thirteenth month of the Western calendar.
Wiktionary 
Origin of Undecimber
-  
From Latin undecim (“eleven”); formed as logical continuation to December, which in its turn is derived from Latin decem (“ten”), as it was the tenth month in the Roman calendar. It's unclear whether the term was ever used in Classic Latin as a name for an intercalary month.
From Wiktionary
 
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