You can cut them in advance and leave them in a pot or bowl of cold water - that will keep them nice and fresh. If you just leave them out they go a bit dry and brownish - not as bad as white potatoes though.
Umm, I reckon they'd be OK the day before as long as you kept them in cold water, in the fridge, in an air-tight container. The only thing I can think of is that they might fade a little in colour (and maybe flavour). I would give it a try with one before the holidays and see what happens.
I live in Scotland so we don't have Thanksgiving - sounds like fun!
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