Formal Night Attire

How many formal nights are there, and how formal are they?

How many?
The number of formal nights may differ by particular sailing or cruise line but here is the norm:
One formal night on sailings of five days or less
Two formal nights on sailings of six to 12 days
And I would suggest contacting the cruise line on longer cruises.

How formal?
Again, depends on the cruise line and in some cases where the ship sails from. If the dress of others is important, you may want to consider a higher end cruise line. While mainstream lines such as Carnival and Royal Caribbean suggest a tux or dark suit and tie for men and evening gown or cocktail dress for the ladies, the enforcement of the rule is not always a high priority. The thing to remember is, this is not just a night to dress up, but a night to get some pretty snazzy professional looking pictures taken. And if the dressing up thing is not for you there are alternative dining options on board.

Tip for the husbands that don't like to dress up: Its only one or two nights. Just think of the brownie points you can earn for it!