X'mas eve would be hectic, I venture, as I imagine a lot of people will be travelling given it's a 4 day weekend in Canada (and southbound at the end of a workweek for any public holiday/long weekend is bad). Wouldn't attempt that crossing in the afternoon (and even Nexus may take 20 minutes) unless you give yourself 2-3 hours. X'mas day would probably be much better -- no shoppers, those wanting to spend time with family probably left on the 24th but some might be getting there X'mas day so I'd allow lots of time still.