Translation:February is not a month of the summer.
´não é´ = isn´t, which gives you your negative in the sentence. I believe that here ´de´ is used because it is referring to summers in general (every summer) which is more appropriate as Feb is never a summer month (in the northern hemisphere). If it were ´do´ it would be talking about a specific summer (like this summer).
I was wondering the same as Amired. "A summer month" would be "um mês de verão" wouldn't eat. The discussion regarding where it's summer and where it isn't doesn't have anything to do with the sentence. I translated this sentence as: "Fevereiro não é um mês do verão." Why shouldn't that be right?
I am not sure about Brazil, which is mostly in the Southern hemisphere, whether February is summer or winter. In English speaking Australia and New Zealand February is a summer month. To us in the UK it is very much not a summer month, not even a spring month, but I agree with the common feeling that we don't say "a month of the summer".
In Brazil, February is most definitely a summer month. Even in the Northeast. Though, the temperature stays warm in NE Brazil, the activities change. (At least, that was the case in Salvador.) There are tons of summer activities, folks travel more and, generally, act like everyone else in the world acts in the summer.