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 honor 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.
December contains the winter solstice in the Northern Hemisphere, the day with the fewest daylight hours, and the summer solstice in the Southern Hemisphere, the day with the most daylight hours (excluding polar regions in both cases). December in the Northern Hemisphere is the seasonal equivalent to June in the Southern Hemisphere and vice versa. In the Northern hemisphere, the beginning of the astronomical winter is traditionally 21 December or the date of the solstice.
Meteor showers occurring in December are the Andromedids (September 25 - December 6, peaking around November 9), the Canis-Minorids (December 4 - December 15, peaking around December 10-11), the Coma Berenicids (December 12 to December 23, peaking around December 16), the Delta Cancrids (December 14 to February 14, the main shower from January 1 to January 24, peaking on January 17), the Geminids (December 13-14), the Monocerotids (December 7 to December 20, peaking on December 9. This shower can also start in November), the Phoenicids (November 29 to December 9, with a peak occurring around 5/6 December), the Quadrantids (typically a January shower but can also start in December), the Sigma Hydrids (December 4-15), and the Ursids (December 17-to December 25/26, peaking around December 22).
This list does not necessarily imply either official status or general observance.
(All Baha'i, Islamic, and Jewish observances begin at the sundown prior to the date listed, and end at sundown of the date in question unless otherwise noted.)
Tuesday immediately following fourth Thursday of November: December 1
First Friday: December 4
First Sunday: December 6
Second Monday: December 14
December 15, unless the date falls on a Sunday, then December 16: December 15
Winter Solstice: December 21
December 22, unless that date is a Sunday, in which case it's moved to the 23rd: December 22
December 26, unless that day is a Sunday, in which case the 27th: December 26