If you are on the ship for more than 9 days you will have 3 formal nights the first the next day after you leave port and the last the penaltimate night before you leave the ship with another in the middle depending on how the days at sea fall, although we have had the occassional case where it was a day when we had just left port. There will be a few days on informal and the remainder casual. On formal nights is the dress up night with gents wearing dinner suits [tux] or a dark suite with ladies long dresses or samrt dresses. On the informal gents with shirt/tie/trousers ladies with trouser suits etc and casual as you would expect but no jeans if you are going to eat in the main diningroom. In many cases you wear what you feel comfortable in don't have to follow the pack 100%.