Like previous posters have said:
1) any day is good because they allow only 1000 guests in the entire park
2) get there very early so that you can pick your time. There is so much more to do at DC you will not be bored no matter what time your dolphin swim.
3) I would not wait until the last day of your vacation to go. When we went, we had a time for 2:30, and did not get to swim with the dolphins until 3:30 because of lightning in the area. I asked what would have happened if we couldn't swim with the dolphins at all that day, and they said that we could come back another day.
I personally think a time right before lunch would be great. Then you get to talk about your dolphin swim while you enjoy a very very good meal!
have fun!