Orange & Fennel Salad is a perfect contrast for Thanksgiving Dinner. Can I slice the fennel today, or will it discolor?

On this, I really would wait. You could slice today, and sprinkle with a little orange juice (or lemon), and cover in the fridge. But since it's only a quick job, I'd find different things to prep today.

Yes you can! Slice the fennel and keep it submerged in cold water in the fridge.