Setsubun is a Japanese festival (Bean Day in the English Version). It is on February 3rd in New Leaf. It Is supposed to celebrate the arrival of spring and to ward off Evil ONI ( devils ).
In Animal Crossing:
When you go to the Town Plaza Isabelle will be there wearing a tiger shirt. She will offer you either a Red Oni Mask or a Blue Oni Mask. There will also be a photo stand of two Oni's one being the blue one and the other being the red one. If you speak to a villager during Setsubun they will say something like "Bad Luck go away! Good luck is inside!" then throw beans at the player.