The Celebration is having a 3 week refurbishment at the end of October concentrating on the soft furnishings etc. At the same time she is having her compulsory 25 year maritime 'MOT' inspection. On captains formal night it is 40% dinner jackets with the vast majority of the rest in suits. You can go informal in the buffet that night if you prefer, smart casual like all other nights in both the buffet and main restaurant.
A tie is really required for the formal evenings in the main restaurant but you can dine in the buffet on that night, where no tie is required. Other evenings in the main restaurant an open necked or 'polo' shirt is quite acceptable.