We've prepared potato salad for 300 the day before. We were just very careful about refrigeration and all that. There were hard boiled eggs in it, so we folded those in right before the potato salad went on the truck.
About the pasta salad...not sure cause I don't know what you're putting in it. Pesto?
The day of. Tomatoes? I think the day of. You could probably cook off the pasta the day before, oil it, and keep it in a bus tub in the walk in. Not sure about the absorption of flavors with that one, though. Good luck.