I will cruising on July 22 board the Norwegian Spirit from New York. With all this free-style going on, could someone who has experienced formal night on the Norwegian Spirit describe what a typical formal dress night might look like.

Formal nights aboard NCL ships are optional. You can dine at the main restaurants aboard the Spirit on optional formal night is resort casual attire, or you can dress up to the nines if you wish.
What I wear is resort causal, slacks and a collared knit shirt.

It can be anything from a little black dress to a ball gown. Here are pictures of what I wore on two cruises:

http://img.photobucket.com/albums/v76/auburnsiren/dress_003.jpg

http://img.photobucket.com/albums/v76/auburnsiren/beardsleydress.jpg

The next cruise I plan to wear this:

http://img.photobucket.com/albums/v76/auburnsiren/bronzedress.jpg

Pick something that will pack well. That is the best advice I can give. Even a very simple but well accesorized dress will do the job.

Thanks for the information. I purchased 2 gowns for the formal nights. I hope that I won't be over dressed on the Spirit. Can reservations be made in the main dinning room?

There is no overdressed. You'll see tuxes and full-length ballgowns here and there. Lots of sequins et cetera.

Seems like we made a reservation on formal night on the Spirit, but I don't remember for certain. It is not necessary but waits can be long. Also, if you want formal photography done prior to dinner, factor that in. Lines for that can be very long.