Ask at the Front Desk - it varies from ship to ship and cruise to cruise - but, hey! you're on a cruise - relax and go with the flow!
Formal nights on any cruise are the second night out and the second to last night on board - longer than 7 days there may one or two additional Formals - the number comes with your final documents or ask your TA.
HAL has its dutch night and also a great BBQ one night at the pool area, I remember steaks on the Veendam. It was sprinkling rain so they shut the retrractable roof, then
they had to open it because of all the smoke from the BBQS.