Translation:October is a month of the year.
A czego innego mógłby być miesiącem?
Would "month in the year" be acceptable?
Well, that's "miesiącem w roku".
But previous translations accepted 'in a year'.
Hey all! Why is 'roku' in locative here? :)
It is Genitive, and of the "noun" and "noun's" usually translates to noun in genitive
"a" was not available for me to select.