Yep, it's a real holiday. I'm guessing Doom's tastes run more to peasant festivals and such - I find it difficult to imagine him in a Hawaiian shirt - but, yeah, it's basically 'Doom is in a good mood! LET ALL REJOICE! Free ice cream cones for all!' Which is why all Latverians dread the next time their leader gets beaten by THAT CURSED RICHARDS, because they know that Doom is going to be in a foul mood for weeks afterwards, which means no ice cream cones.