HI send them as you would if you going to a family wedding i would think that would do ok?
I was on Galaxy 9 months ago & witnessed 2 guys turned away on Formal Night. They were both smartly turned out but no jacket. Reading on other sites, Celebrity appear to be as relaxed about dress code as other lines, so I suppose it's down to the Maitre De on individual ships on how this is enforced. A jacket is the key not necessarily a suit. Celebrity are a 5* line.
When are you sailing? On all sailings on or after August 1, 2008, the dress code will no longer include "informal" nights - There will be "smart casual" nights and "formal" nights only, on all sailings fleetwide. From my experience Celebrity DOES enforse the dress code and I have seen people being turned away from dining room for not having jacket/tie on formal night. Also you are expected to remain in formal dress for whole evening. It only costs a few £ to buy a suit in Tesco or Asda - if you can afford the cruise you can afford to buy this too?