Harry's list is probably reliable. Certainly the second night and next-to-last night are pretty much mortal locks. The middle one can vary, but it's easy to find out from the desk when you board, so it's no big deal.
A lot of people find the middle formal night the ideal time to go to the Olympic, where formal dress is not required, and in fact is discouraged even on formal nights. Jackets and ties are the code there on all nights. However, if you really like to dress formally often, it's easy to plan to visit the Olympic another time.
Im not sure i understood the last message. Are you saying that the middle formal night in the dining room is indeed formal but the Olympic is not on that night....but is formal the remainder of the nights....?
I must of just misunderstood.
We are sailing on 6/21 and are looking to plan our nights deciding which one to make rez for the olympic(we have loads of formal clothes and would rather go formal than casual).