The word December is from the Latin word decem, which means ten.
It seems weird because December is the 12th month of the year; but the old Roman calendar began with March, so December was the 10th month a long time ago in Rome.
weird = strange
In the old Roman calendar there was no January nor February, and December had only 30 days. People then had only 304 days in a year, which makes us very lucky because now we have 61 more days in a year.