This information should be all in your pre cruise documentation. If you haven't recieved this yet, call P&O. They are normally very helpfull.
If this is a two week cruise then there would normally be 4 formal nights. If your agent can't tell you ring P&O and ask. When you get your cruise tickets it will tell you there.
We have done this one.On a 14/15 night cruise you should expect at least 4 formal nights .Also if you are going to New Orleans in Jan /Feb it might be chilly.Also check out the Mardi Gras Parades,.