Saturday afternoon would probably be the best time, since it gives everyone all day Friday to familiarize themselves to the layout of the con and where things are, making it easier to find places Saturday.
Saturday night is the masquerade, and a lot of people may be going to watch that. Sunday is usually super-hectic, what with checking out of hotels and getting home and what not. We didn't even make it to the con at all on Sunday last year. :\
Edit: Later on Friday probably wouldn't be too bad either. Sometime after 5 or 6 pm. n_n
Also, the fountains are probably the easiest place to find, or under the flags. There's a big open 'room' at the bottom of a set of escalators that has 5 or 6 places for photoshoots (the fountains and under the flags included), and that place is pretty easy to find. It's right outside of the videogame hall, I believe.