Is it new ground that you're tilling? If so it will take a few go rounds to get it right. I'd start whenever it's dry enough that the loose dirt won't hold a ball after mashing it in your hands. The spuds should go in mid Feb. through early March.
Back to the tilling: I've got lots of good reasons to NOT till. Come by one afternoon when we can go out back and I'll show and tell. You may find it interesting.