There isn't one. The 4 night cruises had an "optional dress up" night pre-shut down, but I saw no mention of it on our 4 night on the Dream earlier this month.
We were on the Aug 9th sailing and called Guest Services to ask. We were told there is not one. I know of 2 other people on that sailing who did the same. 1 was told Tuesday and 1 was told Thursday. So, if you want to dress up 1 night, go for it when you feel like it.
I second everything that's already been said. There is NO formal night on a 4-night cruise so if you're so inclined you can dress up on any night you want.
If there is a special or themed event going on they'll make an annoucement if they want to encourage you to dress for it. For example, they'll welcome you to dress up for pirate night or Halloween or both if they're the same night like they've been on the Fantasy this month. You can also dress to match the MDR decor if you'd like. When eating in Lumiere's you could dress fancy to match the formal vibe of the decor and look nicer for the photos the photogs come around to take.
Also, read the packing list section on DCL's website. It'll mention attire types based on cruise length and destinations.
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