Theoretically any time you have fertile CM is good. We DTD probably about 3-4 days out from O (I don't track but my luteal period would have to be extra long otherwise) and I know I had fertile CM that day. (And lots of it, so say my panties.)
That being said, the best days are 1-2 days before O. You're most likely to have fertile CM and the spermies won't need to survive that long. Day of works, but the odds are slightly reduced as the egg doesn't live that long and they still need to make it up there.