The best time to visit is while classes are in session. See the campus in action. You can visit classes, you can talk to more students, and you're more likely to meet a professor. Plus, you can get a better feel of how students and faculty interact. This one's up to you.
Some students visit a dozen or more schools during college tours in the junior year or the summer.
Others work with a shorter list. Either way, you should plan on visiting your top two or three schools again during the fall term of your senior year. Read the literature and spend some time browsing the web sites of the schools you will visit. Think about questions you want answered. You'll help yourself with good questions in your Admission Interview, and questions on the tour will help the tour guide get away from a script and give you the inside scoop.