Informal means sport coat attire for men and cocktail dresses for women. Many people were much more casual than that however. A nice shirt or sweater and pants is really just fine, I.M.H.O.
Our recent one week cruise had two formal nights, one informal night, and four casual nights. I hope that helps.
The second night, Monday, was at sea and was the welcome aboard formal night. Wednesday was the second formal night and was the Farewell dinner because Thursday we were in port until 11:00 P.M. Otherwise Thanksgiving would have been the formal night. First and last nights are always casual because of packing and unpacking issues.