As stated on the Silversea website and mentioned throughout this forum, yes, there will be a mandatory “formal night” on your voyage as formal nights are only optional on 7-nights voyages in the Med.
Now, with that said, I don’t think “mandatory” is the proper word to describe formal night since you do not have to wear formal attire if you don’t want to, of course. On Silver Spirit during formal nights, guests may dine in La Terrazza and choose to dress informal; dresses or pantsuits for ladies, jackets for gentlemen (tie optional). This option also applies to Seishin and Stars. You may also dine at The Grill in casual attire.