I am not sure what the deal is - is "to" the absolute synonyme with "jest"?
You have two possible constructions for what I call an "X is Y" sentence:
Czerwiec to miesiąc lata.
Czerwiec jest miesiącem lata.
So the first one just uses "to" and the Y part is in Nominative. The second uses a form of "być" and Y is in Instrumental. ('lata' here is in Genitive anyway, as it is "month of summer).