December is the twelfth and final month of the year in the Julian and Gregorian calendars and is also the last of seven months to have a length of 31 days.

December got its name from the Latin word decem (meaning ten) because it was originally the tenth month of the year in the calendar of Romulus c. 750 BC which began in March. The winter days following December were not included as part of any month. Later, the months of January and February were created out of the monthless period and added to the beginning of the calendar, but December retained its name.

In Ancient Rome, as one of the four Agonalia, this day in honour of Sol Indiges was held on December 11, as was Septimontium. Dies natalis (birthday) was held at the temple of Tellus on December 13, Consualia was held on December 15, Saturnalia was held December 17–23, Opiconsivia was held on December 19, Divalia was held on December 21, Larentalia was held on December 23, and the dies natalis of Sol Invictus was held on December 25. These dates do not correspond to the modern Gregorian calendar.

The Anglo-Saxons referred to December–January as Ġēolamonaþ (modern English: "Yule month"). The French Republican Calendar contained December within the months of Frimaire and Nivôse.