I would visit in the spring, when the 1st years have had 1/2 year under their belt, and the 2nd years are as relaxed as people in a country club.
This of course only applies to people who are not applying this fall.
As for visits, you can do either the official schedule, or if you get to know some of the students well enough, they will give you a private tour or set you up with class visits.
For example, I did both official class visits and unofficial visits to the financial aid office and other offices at Berkeley (because I'm local). for UCLA, because the classes started way too late (in comparison with their R1 deadlines), I got some MBA students whom I spoke with online and in person before school started to show me around. I also just walked around and peeked into different student services offices (career, financial aid, study abroad, etc..) and spoke with anyone who was there. They're generally more relaxed during the summer before school starts, so you may get a LOT of time with them that you wouldn't get during the school year. Stanford unfortunately only has official schedules and do not allow class visits to 1st year classes. The students I met and spoke with there did not really offer to do unofficial stuff, so I didn't push the issue. Kellogg I went completely with unofficial visits, and it turned out to be much nicer than the official ones (where groups of 5-10 people toured around the building, while I got private tours and class visits from 4-5 students).
So yeah, contact people and see if you can get on some unofficial visits. This allows you to sit in classes that aren't listed and may be more interesting to you.
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