I split the difference. I bought pants for a tuxedo and a two pleated shirts with studs, two cummerbunds and a velvet dinner jacket and shoes. The jacket and shoes can be worn with black pants and an open-collar shirt and nobody notices anything amiss!
I usually wear a black cocktail dress for formal nights. I have seen everything from prom dresses and tuxedos to ladies in a nice skirt and men in khakis. Whatever floats your boat (or ship!), grin.
By all means dress formally to celebrate your anniversary if you are conformable doing so. Cruising has changed over the years and is now a little less formal. It has not affected us as we still like to dress up.
Also, welcome aboard the CruiseCrazies ship, slphjp! :smile: