Seadog's memory is correct. Almost always it's on the 5th night of the cruise, which is the 2nd formal night. (Obviously, there can be some variations caused by any number of reasons, such as weather or itinerary change.) But you can pretty much count on that being the case.
45 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!