From Nookipedia, the Animal Crossing wiki
The event plaza (広場, Hiroba, Plaza) appears only in Animal Crossing: New Leaf, and serves as a communal hub of the town. At the beginning of the game, the player who is designated mayor will participate in a ceremony and plant a sapling in the center of the plaza. The sapling will grow steadily based on the amount of time the game has been played, going through five different stages of growth until it ultimately reaches a humongous size. If a player sits on the brick wall surrounding the town tree once it reaches a certain size, he or she will be able to view a list of accomplishments and statistics during his or her time as mayor.
During certain events and holidays and random visits, special characters can be found in the plaza, including Chip, Nat, Katrina and Redd, who will set up tents or stalls whenever they visit the player's town. Additionally, other attractions such as face boards may be present during some events. On some occasions, Isabelle can be found within the Plaza and will give each player a gift, much like Tortimer does in previous games. Whenever Isabelle is present in the plaza, the Town Hall will always be closed.