From Nookipedia, the Animal Crossing wiki
Although people typically celebrate this day in real life when one graduates from a school, the Animal Crossing series Graduation Day holiday is for generic graduations. Graduation Day, or Sudachi-no-hi (Leaving the Nest Day), is the day where the player's villagers commemorate all the hard work and progress the players have made so far in the game. If players talk to the mayor, he will present them with a model of the Able Sisters Tailor Shop for all their hard work.
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