SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08
On my 7 day November cruise departing on a Saturday, the formal nights were Sunday and Thursday (both at sea days).
Generally - the 1st formal night is the night after sailing because it's too hectic to do it the night of sailing with the possibility that people may not have all their luggage in time. And, the 2nd night is usually the night BEFORE the last night - as people are busy packing the last night. That holds pretty true on Princess for both 7 and 10 day cruises.
I cruise the Emerald Princess, Eastern Caribbean on April 16, 2012
On a 7 day cruise your first formal night - The Captain's Gala Welcome Aboard is the first night of your first FULL at Sea Day. Your second formal night. The Captain's Farewell Gala is on the night of your last FULL at Sea Day