If you own Miitomo and have a My Nintendo account, then today is a great day for you. Actually, you can get this reward until May 1st 2016. On My Nintendo you can get a "Mario Suit" costume for your Mii on Miitomo for IOS or Android right now. The costume costs 250 Platinum Points that can be earned by completing basic tasks on the Miitomo app itself.
SOURCE: The Reward