Whenever you want it! You can make announcing your new derby name part of celebrating the achievement of becoming a full-fledged, bout-ready skater, or you can introduce yourself by your derby name the moment you join BCRD. Or you can go by your "regular" name, if that's what you prefer. Your roller derby identity is entirely up to you.
That said, if and when you do choose a derby name, we ask that you choose one that is unique to the league (and try not to choose one that rhymes with another skater’s name-it’s confusing on the track!). Consider some googling before committing so you don’t end up copying a famous skater! Any number not taken by a BCRD skater is fair game. Also, try to keep it PG-13 or you may get edited by bout announcers and rosters.