Can I pre-cut sweet potatoes?
If you want to do as much ahead of time as possible you have a couple of choices.

You can definitely peel the sweet potatoes the day before and store them under water, but I wouldn't recommend slicing them. If they are sliced thinly enough for gratin they're going to absorb a lot of water which will ruin the flavour. You can make the cream etc. mix up the day before though.

The alternative is to make the whole gratin up the day before and keep it in the fridge uncooked and covered. This will mean some of the cream is soaked up overnight and you'll get a very soft finish, but that might be what you want. Sweet potato doesn't hold as much texture as potatoes during cooking anyway so the difference might turn out to be negligible.