If I remember correctly from May it was on Monday night which was Formal night. The party starts about 5:30 I think and lasts about 1/2 hour (might start a little earlier I don't have my book in front of me). Dress goes to all extremes as you have those people already dressed for 1st seating who are dressed more formally and those for 2nd seating who haven't gone to change yet, and maybe some who don't plan on changing. With so many repeat cruisers the parties are getting big and a little less personal. In the early days we would only fill one of the clubs and the captain and officers would walk around and talk to everyone. On our last two cruises (a 4 day and a 7 day) the party went into all three clubs and the captain went to each one to say a few words.
The drinks are free and they do a nice spread of finger foods. Its worth it to at least take a look.