Honestly, I think it's completely random and not something you can ask for.....it's part of the Disney magic....DH and I were the Grand Marshall's in the MGM parade (Stars and Motor Cars) while on our honeymoon. We went to Guest Services, wearing our bride and groom ears, asking for availability at 50's Prime Time for dinner. The CM asked us if we were going to be in the park around 3pm and we said yes. She then asked if we'd be the Grand Marshall's. It was a unique and fabulous experience, mostly because it was so unexpected.