Campus is very compact - you should be able to see it all on Sunday easily. You can walk from any point on campus to another in 10 minutes or less, always, so seeing even most buildings on campus would take maybe an hour max. Going to campus on Saturday in addition to welcome day would be a bit of a waste in my opinion. So, despite the request, I’d recommend exploring neighborhoods around Northeastern to get a feel for the area as well as looking at campus. Fenway is a big food neighborhood in the area and Newbury St / Boylston has a range of shopping and food, which is of course next to the prudential center. Copley Square is also a nice area along Newbury / Boylston. The best route would probably be to start on campus, walk over to Fenway, take the T from Kenmore to Copley (only 2 stops), then walk up Newbury and eventually into the prudential center and back to campus (via Huntington on the other side of the pru). That’d be a nice loop where you’d see cool areas, good food options, and can get a feel for campus before/after the loop. On campus ISEC is the newest building that’s probably worth seeing which you may not see on Sunday, though likely will. West Villiage is the main on-campus housing area, and you can check out East Villiage (honors housing) easily while there too. You can walk into Marino to see the gym as well.
It looks like there will almost certainly be snow on the ground, so do bring boots and a warm jacket 
Hope that helps!