Where you live relative to the college, what you can afford in terms of transportation and hotel costs, and the complications of missing work, school, athletic practices, and play rehearsals will all influence when you choose to visit schools. You may find that the summer visit is the one that best serves your needs. Once you have narrowed down a list of choices, plan on making formal visits, lasting at least three hours each, to the schools to which your child is seriously considering applying. If your child is a junior or senior in high school, that visit should also include an on-campus interview if the college provides the opportunity. It is important to be aware of the college's policy regarding timing of interviews.
Nothing is cast in stone, however.
If your niece's wedding brings you to Boston from your home in Minnesota in the fall of your son's junior year, some gentle persuasion may grant him an interview at the college of his dreams. The standard college visit includes a campus tour, which takes about an hour and covers much ground. Wear comfortable shoes and bear in mind that many campuses are quite hilly. Many tours point out the prevalence of security on campus, which doesn't mean the school is a war zone. Don't be dismayed by muddy spots and construction.