Suggest you ring Cunard HO in Southampton (or Carnival as it now is) It's a local call for you or look on their web site and it may be in the Cunard brochure. Last resort is to talk to your cruise agent.....Falan Dow.
There is usually three formal, one informal and two elegant casual. The nights are listed in the brochure.
Every night but the last night is formal on trans atlantic (last night still requires jacket at dinner.
Last year i had 3 Formal ,1 informal and 2 Smart casual.