Congrats to you - junior on planning ahead and considering NU. I have a D alum and also a current Senior at NU.
Google Is Your Friend - Look at School Calendar for specifics: http://admissions.northwestern.edu/visit/plan-your-visit/index.html
So plan ahead and plan to sit-in a class in your interest area. It may or may not be extremely cold in Nov-Dec - sometimes it’s still a nice crisp day - but be prepared if you are from CA or FL. Good Luck!