In old Japanese language, December was called "Shiwasu."
The original meaning is "It's as busy as even high Zen priests (=師) run (=走)". New Year's Day is the most important day to start with a pure condition. So Japanese don't want to carry jobs or issues over to a new year so we get busy in December to finish them. That's why even high Zen priests, who are usually calm, run in this month.
Japanese people have a custom of doing a general year-end cleaning to greet the New Year. I feel busy for planning to do a big (not so big, bigger than usual) house cleaning and to prepare for New Year's Cards.
In addition, although only 1 % of Japanese people are Christians, we enjoy Christmas activities in order to feel the romantic atmosphere by exchanging Christmas presents, having a party and decorating streets with illuminations.
December is super-busy for most Japanese people like in the old times.