Will waffles be fresh if I make them the day before?

Your choice: you can make the waffles the day before, cool them to room temperature and store them air-tight at room temperature or in the fridge, or pop them in the freezer. You can employ your toaster to defrost and re-heat the frozen waffles; otherwise, your idea to heat them on a baking sheet in the oven is a fine one. I wouldn't put them in an oven any hotter than 250 degrees Farenheit. Enjoy your party and Happy New Year's Eve!