Apparently in (some regions of?) Bavaria, too. But as far as I know that's only dialectal and it's still masculine in standard language, right? Or can you actually use das Monat in formal texts in Austria? In German standard language it sounds plain wrong...
I just looked it up in the official Austrian dictionary and it says that "das Monat" is colloquial. But I would probably write it even in a formal text because I wouldn't even think of the possibility that it could be colloquial. It's just normal for us.
At the beginning of this unit, it is spelled Monate, with an e, is this something different, or a mistake?