Formal nights are also referred to as Celebration Nights or Gala Evenings, depending on who you are cruising with.
Formal nights vary between Cruise Lines and Itineraries. It is best to always check your pre-boarding documentation for any themed or formal nights on your sailing.
As a general guideline, there will be approximately:
1 Black Tie Evening during a 2-4 night cruise
2 Black Tie Evenings during a 5-7 night cruise
3 Black Tie Evenings during an 8-10 night cruise
4 Black Tie evenings during an 11-14 night cruise