User blog comment:Hfc2x/Main story timeline/@comment-25358468-20140501142532/@comment-10733658-20140501200428

I've always assumed it wasn't BRA's decision, but rather the orders they were given. Attack once they graduated and were given full access to the Wall as soldiers, without any suspicion. The timing during the day was likely because they waited until the Survey Corps was far away, and therefore unable to join the battle.