A couple of times during the cruise, the ship hosted celebration nights. We even got vouchers for a free glass of champagne.
Many dress up including the men in formal jackets or dinner suits. And yes, the restaurants will enforce the dress codes.
During our cruise, most evenings were casual then once a week, a formal celebration night. While we did not participate, many did with the Horizon magazine advertising any dress codes.
The restaurants participating host a special 5 course celebration menu, hence requiring a dress code. For those not participating, some of the buffets and restaurants are available. Alternatively, you can use room service.
https://www.pocruises.com/what-to-wear
https://www.pocruises.com/onboard-activities/dining/celebration-night
