On NCL formal nite is optional. You have the chioce of formal attire or remaining in resort casual.
On my last cruise, I saw few Tuxes & Gowns. Most women wore cocktail dresses---------men in dark suits.
This is the main feature which draws me toward NCL. No one will criticize me for leaving formal clothing behind. Cocktail dresses for me, and a nice suit for DH is as good as it gets.