We don't do BBB but some people try to tie to formal night; others do it before a princess gathering; or any day and let the girl(s) dress up for the afternoon and/or dinner. Some girls don't like swimming after BBB, so you might not want to do it right before a port adventure to the beach or such. It's really entirely up to you when it will fit your family's plans best -- maybe take a look at your port stops and what excursions you would like to do and then go from there.
Pools onboard get very crowded, especially in the afternoon on sea days. We don't spend much time at them. Mornings, during port stops, and during dinner can be the lower crowds at the pool and Aquaduck.
Enjoy your cruise!