The first formal night is always a Black and White Ball, so try to ensure one of your evening dresses is black and/or white. On our recent 10-day Caribbean cruise we also had a Masked Ball (masks on sale on the night) and one other formal evening. If you are 2 single ladies, dance hosts are provided - BUT they tend to be ancient.
You will have at least two formal nights when most people are in formal dress and it is almost impossible to overdress!! Unlike many cruise lines that have formal nights that are very lack lustre and going through the motions these are for the real thing ... so get out your best glad rags and enjoy!
Cunard send you an itinerary before you go detailing how many formal, semi formal or smart casual nights there will be
Formal Men dinner suit, ladies long dress or skirt
Semi formal Men Jacket & tie, Ladies smart dress, skirt or trousers
Smart Casual, Men Jacket no tie, Ladies smart casual, trousers, dress or skirt
Formal nights usually have a theme ie black and white ball
Hope this helps, have a lovely time
Thank you for your answers I will let you know how we get on.