As you know, there are 3 formal nights on your cruise. I'm just "guessing", but I'd say day 2 or 3, and day 16 or 17. This is just an "educated" guess, but hopefully better than nothing !
The formal nights will be on "sea days". These longer cruises are so "unusual" that you may not be able to find out until you board. I'd suggest you go to the Specialty Restaurants desk as soon as you board and they'll be able to tell you exactly and you can book what you want.
Sorry I can't be of more help ,
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