May I successfully roast sweet potatoes a day ahead and reheat to Serve?
absolutely - just don't cook them all the way the first time so that they are not overly done when served. Also not sure if you are talking about whole or cut potatoes.
Agree with Liz B.
To her point about what kind of potatoes, whole or cut. And, implicitly, how you plan to serve them.
For cut roasted sweet potatoes, they work fine if done once, held in fridge, then finished or done a second time.
For whole roasted sweet potatoes, especially if you intend to serve them mashed, I would follow methods for making (regular) mashed potatoes ahead of time...for example,
If you plan to serve in the jackets, be aware that roasted sweet potatoes "weep" quite a bit when stored for any length of time . . . syrup oozes out, even of whole ones. That's not a reason however for not roasting in advance. ;o)
P.S. Incidentally, I noticed in a recent issue of Cook's Illustrated that their testing shows that the best way to roast sweet potatoes is to microwave on high for 10 minutes and then put them in a 425 degree F oven for 45 minutes.