How do I find out when grad schools host their open house? I'm looking to tour a few grad schools in the near future, but I can't find any dates for tours for any of the schools I'm interested in.

The school I attend has tours for potential grad students, but I'm not sure this is a thing all schools do.

Do grad schools typically have days where they hold tours?
Should I contact the schools I'm interested in? If so, who should I contact?

(I have not yet applied, I'm about a year away from that.)
If you are in the middle of the graduate school application process, they will generally tell you (upon acceptance) when their open house is. If you are thinking ahead (e.g., to next year), I would send an email to the office of graduate admissions, or perhaps to the chair of graduate admissions for the department you're interested in. They may not have organized large-scale tours for prospective graduate students who have not been admitted, but I imagine they might be willing to work with you to get you on a campus tour, set up a meeting with professors whose work you are interested in, etc.