Unlike other Auras, an Aura with bestow isn't put into its owner's graveyard if the enchanted creature leaves the battlefield or becomes an illegal creature for the Aura to enchant. Rather, the effect making it an Aura ends, it loses enchant creature, and it remains on the battlefield as an enchantment creature. It can attack (and its abilities can be activated, if it has any) on the turn it becomes unattached if it's been under your control continuously, even as an Aura, since your most recent turn began.