Watch your weights handled and growth as indicators. I was never one to have a specific 'arm' day because my arms always got hit well enough from grouping them with back or delts, and I was able to grow with this approach.
Other people will tell you that their biceps and triceps can recover quicker and as such can get direct stimulus on one day, and indirect on another. Be objective with yourself. Also, pay attention to how soon you will train back again after your bicep day. if your back session suffers due to remaining bicep soreness, then it may not be the ideal approach for you.