From Nookipedia, the Animal Crossing wiki
Weeding Day is an event that happens on September 3rd in Animal Crossing: New Leaf for the Japanese version and the last Friday in April in the US and European version. Leif will hold this event when the Garden Center has been built on Main Street. The event is described as a day for the town's villagers to come and remove weeds from around the town, though this is up to the player.
On Weeding Day, Leif will be at the Event Plaza where he will tell the player how many weeds (clovers do not count) are in the town, when all these are pulled, Leif will give the player one of twelve pieces of furniture from the Flower set. Certain items from the Flower set, however, are obtainable only if the player removes weeds in other players' towns.
If on Weeding Day, the player has no weeds in town, Leif rewards the player with the Happy Generosity Award and unlocks three new public works projects - the round, square, and tulip topiaries. Additionally, he gives the player a flower bag containing a hybrid flower.
Leif will give furniture from the Flower set after all the weeds are pulled. Items are listed below.