Thanks, I wondered about that. It would be like saying "the grandson is coming in December", depending on the context, it can be about a yet to be born child.
when talking about days you use "na"/"no". but when talking about months or year you don't use de definite articles ("a"/"o") so it's "em"
ex: meu neto chega na quarta.
meu neto chega no dia 21 de janeiro de 2016. meu neto chega em janeiro de 2016. meu neto chega em 2016.