Is "March is a month in the year" wrong? It's a pretty normal way of saying it in English...
It sounds to me like just another way to say the same thing, so you might try reporting it.
Does this mean that there doesn't need to be a Polish bit saying "of the"? (It's just inferred from the context?) Thanks!
One of the most important functions of the Genitive case is performing the function of English "of". And "the" is not-existent in Polish anyway.
Is it miesiącem (instrumental) roku (genitive) ?
How is it translated into "month of the year" by using declensions?
Well, if you consider "month of the year" as a standalone phrase it's "miesiąc roku" (roku in Genitive, because of 'of'), and in the sentence, "miesiącem" takes Instrumental, but luckily "roku" stays in Genitive.