This is the 7669th most frequent Japanese word.
十二月
"十二月" means "December."
The word '十二月' is used to specify the month in a factual and declarative statement.
十二月は冬の始まりです。
December is the start of winter.
Here, '十二月' identifies the month of a personal event.
私の誕生日は十二月です。
My birthday is in December.
In this sentence, '十二月' references a time frame for the described occurrences.