That depends on the ship and the itinerary. They definitely won't be the first night or last night, since the first night people are usually busy unpacking, and on the last night, people have to pack, and their formal wear might already be put away by the time they get ready for dinner.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)