Yay Day is an event in Animal Crossing: Wild World. It is a holiday where the player and all their neighbors compliment each other. Strangely, the player may even write insults, though the locals don't seem to react any differently. It seems that any form of "Compliment" whether it is, in fact, an insult or not, seems to increase an animal's friendship towards the player, even if it is an insult. After you tell them it they will say (Your compliment) I like that! (or something like that) In the days leading up to the festival, villagers will be practicing their compliments when you speak to them.
Further on in the year, some villagers will mention the compliment you told them on Yay Day. Most of the time snooty and lazy villagers will bring them up. Most snooty villagers say: "(Your compliment)! Why does that sound so familiar? Oh yeah! That was the cheesy pick-up line from like, a year ago!" Also, the lazy villagers say: "(Your compliment)! I can't wait to say it to my crush!"