One of the cool things that Animal Crossing New Leaf offers is exclusive downloadable content (DLC) that you can receive via SpotPass. Currently, you can get a Rainbow Screen from Pelly (or Phyllis, depending on the time of day you visit) in the Post Office on Main Street. To get the Rainbow Screen, you have to ask Pelly/Phyllis about a gift, then agree when they ask you if you want to connect to the internet to receive limited time items. Once they finish searching, they'll apologize for keeping you waiting and will hand you your gift.
This Rainbow Screen DLC is only available for a limited time (until June 30th, 2013), so if you haven't gotten it yet, head on down to the Post Office and collect your gift!