Translation:Mia kuzo alvenos la naŭan de decembro.
Can I just use number like the 9 in English?
Perhaps 9an or 9-an, but no. You can't just write "la 9 de decembro."
Got it, thanks :)
Technically, "la nauan de decembro" is not a direct object of "alvenos" in this sentence (it's a time adverbial). I'm a bit confused about the use of the accusative form here.
You can think of -n as taking the place of on the 9th... or you can think of it as "that's how German does it." It's one of the many uses of -n in Esperanto.