Informal interviews can mean a variety of things. For example, some people get phone/skype interviews before getting an official invite just to clear up stuff from their application, talk more about their background if they didn't come from a psych undergrad, etc. A lot of people will get official interviews without having one of these informal interviews. Also remember that at most schools, interview invites go out at different times and some people won't hear back until a month after other people hear back. I'm freaking out too, but it's nice to keep this in mind!!