January was made the first month around 153 BC. Before then
January was the 12th month and February was the 11th month.
January is named after Januus, who is two-faced so he can
look at the year past and the year to-come. I think that
the idea was to get the new year running after the solstice,
which was then the 25th day of a 29-day December.
Between then until Caesar changed things, the months were
January February March April May June Quintilis Sextilis
September October November December, alternating 30 and 29
An extra month, usually called Mercedonius, alternating 22 or
23 days, was supposed to be inserted six days before the end
of February every other year.
For political reasons the people who were supposed to proclaim
the extra month, often did not.
By 47 BC (whatever it was called) the calendar was a 100
days early, so Caesar, with the advice of Sosigenes of Alexandria,
put 15 months in 46 BC: January February Mercedonius March
April May June Quintilis Sextilis September October November
December Undecember Duodecember.
In 45 BC things were supposed to be fixed, with the month of
Caesar's birth renamed July, and the days of the months
31 29 31 30 31 30 31 30 31 30 31 30, Every 4 years an extra
day was to be inserted before the 25 of February.
However they started by putting in a extra day every three years.
Augustus fixed this by saying that until they caught up there
would be no leap years. So there were no leap years between 12
BC and 8 AD. He renamed Sextilis after himself, took another
day from February, and changed the days in the months after
Monday, December 31, 2007
Why December is the Twelfth Month
A post by Wil Baden in comp.lang.forth, dated 1999:
Posted by Jenny Brien at 10:00 AM