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.