We have done it twice. We really enjoyed it.....we were brought up to the top of the lodge to raise the flags, wonderful view!
Ask when you check-in. We asked on our last trip (wanted my brother's family to be able to do it), but it was all booked up. The only times we have been lucky enough to book a day has been on longer trips. It seems like it has been booked at least a week out, but you never know!
FYI, twice we have encountered CM's at check-in that had no idea what we meant by "flag family". We just asked another CM. They have a 3 ring binder that they keep the sign-up log sheet in behind the counter.