It is typically on night 2 (the first sea day). Many of the characters will be in their formal outfits for pictures, and their will be photo backdrops of the ship as well as a professional photo area for formal family pics. as PP said, you can dress up in a Tux and Formal gown, suit and tie and cocktail dress, or wear jeans and not really participate (you will see all varieties). For us it is great to get family pictures and it feels special to be able to dress up with my girls.