Usually "on hold" once you interview is equivalent to being waitlisted. Each school has different terms for the waitlist and different ways of dealing with it. Some schools will revisit the list after they are done interviewing and let some people in and then officially put the rest on a "waitlist", some will rank the list, etc. They may start pulling people off before they finish interviewing or after. Depends. Basically what it means is that now you have to wait for a while. Usually sending the school updates throughout the year is a good idea, because it shows that you are still interested in them. The letter you received may give you some insight into the process at each of the schools, or you can call the admissions offices and find out.
Although some people do get in on Oct. 15, the vast majority have to sit around and wait at some point in the process. It's the worst part, but at least you know you're not alone. Good luck!
