Magic Kingdom's flag retreat ceremony is held at 5 p.m. at the entrance to Main Street U.S.A. Members of the Armed Services and Veterans can volunteer to be the day's special guest by visiting guest services in the morning.
This ceremony is held everyday at 5 p.m. If you can take the time to see it, then do it. It's a touching experience.
Source: Undercover Tourist
A Veteran is selected each day to participate in the ceremony.