La-Di-Day is an event featured in Animal Crossing: Wild World, which occurs on every second Saturday of every odd numbered month. On this day, all of the townsfolk will start to sing melodies, and in the end you get to choose whose the best singer. The best singer's song will then officially be the town's tune, though if you later feel like someone else's song is better, then you can just choose his or hers instead.
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When?
Every second Saturday of - January, March, May, July, September, November