It's a suggestion, as the theme that night is "Formal Night".

There WILL be people onboard in tuxedos and evening gowns. Some people like to go all out.

It's totally OPTIONAL though. In lieu of dressing formal, guests are welcome to dine in clothes that meet the dining room dress code every night of the cruise. (No jeans, cutoff shorts, tank tops, or swim attire, no flip flops)

We didn't go to the dining room on Formal Night. We were not in the mood to dress up and didn't want to feel out of place. We ate dinner elsewhere that night.

Jackets are only required at Remy, from what I recall.