Well now I think these are two different things. The website and information I provided was for a theme cruise, which is the whole cruise.
However, RCCL also does a Mystery Dinner Theater on ships with Portofino Restaurant with an itinerary of 7 days or more.
From the FAQs: How long is Mystery Dinner Theater? Please allow three hours for dining experience. Please plan on being seated with other groups or guests. What are the hours for Mystery Dinner Theater? Event begins at 7:00 p.m. Hours may vary according to the ship and itinerary.