The summer before the final year of undergrad would be a great time to visit because in most cases students don't have a lot on their plates (at least relative to other times of the year) at that time. The story might be a little different when you've got three assignments due and the GRE to study for and a job and ...
Remember, the main purpose of visiting a potential graduate school is so that you can make the assessment as to whether that school will be the right fit for you.
Going a little earlier will give you time to mull over what you saw, the interactions you had, read up on any projects that you might discuss, and generally sleep on the decision well before anything is due.
Yes some professors will be busy. Some will be away. But that will always be the case.