From Skip's post above...
"Purple top turnips will stay firm and crisp into winter, Radish will not. I'd say Radish will have a preference over turnips into November and the opposite after that (when radish starts to rot and turnip sugar & taste is better).... I still see them eating turnips even in September or October though."
I very much agree ^^. To add to this from my experience. I never had trouble with deer eating the turnips like many others have reported. I don't doubt their reports, but it is odd that some deer appear to take to them right away and others seem to need to develop a taste for them. I will say though that if you have trouble with deer getting started on them...I would shift the ratio of the mix to be more towards radishes than turnips, maybe like 75% radish, 25% turnip. I think the deer will go after the radishes first from what I have seen.
I have had firm radish bulbs on into mid-December some years and other years they are mush in early-to-mid November. The key is when you get your first killing frost or two. Worst case - say you have good radishes and there is a hard freeze or two in early October, but then it warms back up and maybe runs a little warmer than normal after that. You will not be able to stand the smell of rotting radishes by Thanksgiving...and the deer pretty much avoid them.
Best case - no real hard freezes until mid-to-late December and assuming that all of the radishes are not otherwise consumed, you will have fairly firm radishes still up to near Christmas. Radishes are much more susceptible to the effects of freezing temps and are better in September and October, turnips are better though later, like December and January. What's that, you say I skipped November?
That's where it depends based on freezes in October. No early freezes in October = radishes still more preferred in Sweet November. Early freezes in October = RRAS(Rotten Radish Avoidance Syndrome) is in evidence and turnips become more preferred.