Much depends on the words your boss uses, and whether he tells you in person or writes to you. In either case, simplest is best. It may well be that all you need to say is: 'Fine'.
If you want to be a little more formal with your boss, then try: 'That date is convenient.' Using accept or acceptable carries a slight suggestion that some days might not be acceptable to you; I imagine this is not the case.if your boss is telling you when to go.