SMG, I don't know what cruise line you are sailing. So I am only speaking about Carnival.
Most sailings have 4 dinner times for the main dining rooms. Usually Main 6,6:30, Late 8,8:30.
Not to dispute Sandy, but to my knowledge the Formal dining room for breakfast and lunch as it says Open Seating refers to it not being assigned seating, not what time you are expected.
The casual Buffet's are come when you want, sit where you want etc. during their service hours.
I no longer discuss dress for the Main/Formal dining room
As Sandy said there is a wealth of info on the cruise lines sites. And I am only referring to Carnival.
Best to you for a wonderful cruise,