Erm, I thought that's what I just did.
I've since figured iit out, I think. The insistence on not confusing "finish" and "end" is that while you can use them interchangeably in English in this context, you can't exchange acabar and terminar the same way.
Oh, I meant if you reported it under "report a problem" as well as in this discussion forum. That is the way to go if you want them to fix something. :)
I think your answer should have been accepted.
This thread will explain a lot: http://www.duolingo.com/#/comment/96369. It includes a link to a video that explains the difference between end and finish in English. In short, while it might be grammatically correct to say May finished, it isn't what we would say in English.
I wrote the same as you and got marked correctly so I guess the bug has been fixed and you were correct.
They usually require the closest translation, and one that is closer to your suggestion would be: "Hoje não é o fim de maio". I hope it helps! =)