Why not say "a day" or "a year", but "a month"?

"Ge" is a quantifier, and words should be used between numbers and nouns. But "nian" itself is a noun and a quantifier. Therefore, the quantifier "ge" need not be repeated. Just use "one year".

Example: This year is really difficult! -quantifiers.

Where did you spend this year? -Nouns refer to Spring Festival, so quantifiers can be added.

Because there are twelve months in a year, the word "month" is used to indicate the length of time, and the quantifier "January" is removed to refer to January, so the meaning has changed.