Congrats on your interview! I interviewed on November 11 and found out within a month whether I was accepted or not (around 12/9 I believe). The day consists of meeting with the other prospective students, being introduced to the faculty and staff, and receiving some information about the program. Then, half of you break off into group interviews while the other half hangs out in the room and asks current students questions about the program. Then everybody swaps. After that, you just have a quick one-on-one meeting (less of an interview, more of a review of your application and an opportunity to ask the faculty some questions) with a faculty member. Then you're done! It was pretty laid back and welcoming. I loved it enough to pick it!
Hope it all goes well, and maybe I'll see you at GW in August! Let me know if you have any other questions.