December weather averages:

https://weatherspark.com/m/26197/12/Average-Weather-in-December-in-Boston-Massachusetts-United-States

It can be really beautiful or super miserable if it is 38 and raining...

Big snow month in Boston is usually February, although we can certainly have some snow any time from November on -- until at least April ( there have been some later, but that's truly atypical)