The dining rotations are set for each cruise. Every reservation is assigned to one of the rotations. You can call
DCL and request a specific rotation, but it's not guaranteed that you'll get it. Most time such requests are honored, but not always.
You can only request one of the already set up rotations, you don't make up your own rotation to request. For example, a 4 night Dream cruise typically has a 1223 rotation. The order of the MDRs is Enchanted Garden followed by Royal Court followed by Animator's Palate. 1/3 of the passenger complement is assigned to begin their rotation in Enchanted Garden, another 1/3 in Royal Court and the last 1/3 in Animator's. So the rotations for a 4 night Dream cruise, typically are ERRA; RAAE; AEER.