As it says on Celebrity's web site:
pants suit, skirt and blouse, or casual dress.
On informal nights, men are requested to wear a jacket, shirt and slacks, No jeans, while women may want to wear a suit or dress.
For the formal nights, including the Captain's Welcome Aboard Cocktail Party, dinner on the second night, and the Captain's Farewell Dinner held the next-to-last night of the cruise; both men and women may prefer more dressy attire, such as an evening gown for women and a tuxedo or dress suit for men.
Formal night means dinner suit, which obviously includes a dress shirt & dickie bow for men & a long dress,or top & long skirt for women. Celebrity, in their brochure, make great play of formal meaning formal, very dressed up & I for one endorse that, cruising is the one holiday left that one can dress up like 'days gone by'. But they do nothing to prevent the occasional person who think dressing up is a fresh polo shirt & clean trainers from entering the dining room. On the Summit, most guests will dress up as I have described though. It is, in my opinion, a great night, a wonderful sight to see all in their 'finest'.
Formal is ok for men to wear a dark suit and tie .. flying from Australia to get ships in other parts of the world can make packing difficult due to weight restrictions. As a guy I will have a dark suit and a range of ties and shirts in different colours so that I do not look the same each time.
The ladies seem to have lots of mix and match with long skirts etc and jewellery. Some I know take the wardrobe of dresses .. they live in England so easy.
It's realy a case of looking nice.
Living in the casual world of today ... its a joy to sit in the coctail lounge and see the ladies looking fantastic ... no jeans/T shitrs and thongs. Plenty of time to be casual in the day.
As a man I also take a blazer and sports jacket for other evenings or cool climates to wear.
Hope this helps ... I am sure you will enjoy Celebrit.