In english you can say with the least days, without it changing the meaning. If you want to use the word "fewest", you have to use the definite article, "the fewest", (which is not accepted as of 17/2 -14).
No, if you set the "os" article, you're gonna change the sense of the phrase. The "os" on the phrase will mean that february is the month with the smaller days. The correct answer is Fevereiro é o mês com menos dias
I completely agree. "Fewer" would only be correct in the context of something like "Out of February and March, February has the fewer days". I may be cr*p at Portuguese, but I learned about "few, fewer, fewest" at my mother's knee.