I’m looking to visit in November or December. Is this a good idea? When are students on campus? What are some suggestions? I can only stay for one day.
Current junior btw.
Go as soon as you can. The weather can start to get wintry (near freezing temperatures with rain/sleet/snow) around Thanksgiving. Also, the days get really short after the clocks go back.
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!
Suggestions: Good locations for student watching (always entertaining) include Norris University Center, the Lakefill (but maybe not in November/December, since it’ll be a bit cold), and the lobby of the Ford Building. If you want a bo$$ view of the lake/Evanston/Chicago, go to the top floor of the parking garage of the visitor center building. Also, if you like jogging at all (or taking very long walks), I suggest you go on a run along Sheridan from South Campus to North Campus, follow Sheridan to where it merges into Ridge, and then follow Ridge to the giant Bahai temple.
And yeah, sit in on a class in a subject you’re interested in. Have fun! 
Try to go before thanksgiving! many classes will not be in session the week after thanksgiving break and the next week after that are finals.