Like many countries, Japan uses a 12-month calendar. The names are very simple. January is literally “Month one” 一月, February is “Month two” 二月, etc.
However, before the Meiji Restoration (mid-1800s) it was common to use an older 12-month system. These months’ names referenced the weather and the seasons (similar to the French Revolutionary calendar).
December is 師走.
The kanji:
師 = teacher or religious mentor. In this case it means “monk”.
走 = running
In December, monks are super busy preparing for the New Year’s festival!