"His birthday is in December."
Translation:Jego urodziny są w grudniu.
W grudniu są jego urodziny not accepted - is this an unnatural word order? In English, "in December" would go at the end, but most Polish sentences on Duolingo with this kind of time expression seem to have it at the beginning by default, and I gathered that was the place to put it unless we want to emphasise the time for some reason.
In Polish, the old information (aka topic) usually comes first and the new information (aka comment) usually comes last.
Jego urodziny są w grudniu.
We are talking about him or his birthday and then someone mentiones when his birthday is.
W grudniu są jego urodziny.
We are talking about what's coming up in December and then someone brings up someone's birthday.
I think the second version is fairly probable, therefore added.