Sorry I'm late to the party on this! I always forget about posting on CosCom until about a month before a con.
Last year we had two evening shoots, one on Friday behind the main atrium fountains, and one on Saturday at the gazebo. I would suggest either of those again for locations, at least, since I've found the one think the Gaylord lacks is sizable dead ends perfect for photoshoots. Of course, the space behind the fountains tends to be noisy and crowded in general (though, in my experience, the majority of people that congregate there seem to be really good about respecting shoots by at least not getting in the way), and the gazebo is *the* prime location for shoots in the Gaylord, so I would suggest against taking a long time there in the interest of being fair to everyone that wants to use it.
Outside of that, I would actively suggest against anything outside because National Harbor is super windy and that's going to kick a lot of costume ass. (My Aya outfit of cons past was particularly susceptible to the wind, which is one of the greatest injustices in history.)
Days and times? One Friday, one Saturday again should be good, though perhaps having one in the afternoon rather than the evening would be better so we don't have to deal with a nighttime backdrop twice.