Yay Day is a special event in Animal Crossing: Wild World for the Nintendo DS that takes place on every fourth Sunday of every other month. Of all the special events in the game, this one is arguably the least special since it won't reap you many rewards other than an improved (or the exact opposite) friendship with the villagers. They're all extremely happy on Yay Day, and will compliment you. You'll be able to then compliment them back, though you'll have to remember their personality and try to pick what best fits them, or else they'll be offended. Later on they'll ask you if you meant what you said - pick the right answer accordingly.