"May is your month!"
Translation:Mai ist dein Monat!
In German, you would rather say "Der Mai ist dein Monat!", if you wanna express yourself properly in German. It is accepted as correct, so no criticism towards Duo, just a hint for those who learn German here.
Why is it not deinen Monat, given that Monat is masculine?
Because that would be the accusative case, but there is no transitive verb with a direct object here. ("your month is been by May" makes no sense, for example.)
sein "to be" and werden "to become" take a predicate in the nominative case, not a direct object in the accusative case.
Das ist dein Monat. Er ist ein guter Vater geworden.