For the best result I suggest on holding off on this until you have completed ALL of the other playthroughs (Tohri's ending (#14) is NOT required), so you will also get the BBL Epilogue after the end.
When you start a New Game, you will get a message that you can take this chance to fulfill a promise from long ago, or you can live a normal school life this semester.
When prompted with the choice, choose "Fulfill the promise" to start the BBL playthrough.