If it's the same as it was a couple of years ago, you sign up ahead of time because if no one signs up they cancel it. I think it's every day, or at least several times per week (I recall having a choice of days). You meet at the GCH DCA entrance and leave your phone in the room because it's not allowed on the walk. You have to be an on site guest but it can be any of the three hotels.
Details might have changed since then, so ask at checkin. I didn't end up doing the walk, but did ask about it.