The letter I received didn't mention any specific visitation dates by program, it just said to contact a specific professor about making arrangements to visit. I applied for Physical Chemistry though, so maybe it's different.
I don't see any way to edit, so I'll just add another here.
University of Minnesota, Twin Cities: April 1-3 (guess they don't want you to visit while it's freezing out)
I received a letter from the University of Chicago, then the next day I got a phone call and email from a Professor I mentioned in my personal statement.