Why is December the twelfth month when deci means 10?

Answer 1

December means tenth month in the ancient Roman calendar.
January and February were added in 700 BC by Numa.

In the old days of the Roman calendar, the winter period of January and February was effectively monthless, and the year started in March. Therefore, in the old Roman calendar, September was the seventh month (septem = seven), October was the eighth month (octem = eight), November was the ninth month (novem = nine), and December the tenth month (decem = 10).

'Effectively monthless'? Not sure what that means. I am told that comparative mythology suggests that the reason why the Dark Ages were so called was because there was a close encounter with a heavenly body which blotted out the sun for decades, and which shifted the earth onto a larger orbit, with more days in the year, thus creating a demand for two extra months in the year. Extraordinary as this may seem it rings true for me. I don't think the Romans would leave sixty days in the year outside their calendar. They had an empire to manage. The reason why 60 days were outside the Roman calendar was that these days didn't exist, until this cataclysmic event.

According to The Bible the month of Nisan, which is the month containing Passover is the first of the year. This is also equivalent to the solar month of March. Since Passover must always be the first full moon after the spring equinox, the lunar/solar calendar in use in biblical times was adjusted so that the first month always started at the new moon before the spring equinox.

BTW this implies that the superstition of the year being 60 days shorter could not have occurred as the Jewish calendar was the same in Roman times as it is now.