"Tours" are different from "open houses". On "tours" there is zero chance you'll meet or hear from any faculty. At an "open house", there is 50% chance you'll hear from one or more faculty, and 25% you can actually talk to faculty during the reception. It would be very rare (and delightful) that a graduate applicant "open house" would have most or all of the faculty.
Regarding "open houses", there are three possible methods to find out about them.
First is to look at the Department's web site, and check under "Events" and/or "News". (Sadly) They might only give 2 weeks notice for "open houses".
Second, the department might have a mailing list where you can add your email address to receive regular (daily or weekly) emails on upcoming events.
Third, the open house may be not associated with the department but instead it might be hosted by the School, College, or equivalent. You may need to add your name/email to a mailing list for these events.
Remember: Departments are notoriously irregular in their "customer service" in this regard. Almost no academic departments have incentives to perform their operational processes effectively.