All this month EB Games will be handing out legendary Pokemon over Wireless Mystery Gift.
The first three weeks, this will involve the alt-color Legendary Beasts - one per week. Right now is Raikou, with Entei and Suicune to follow week by week. Starting the fourth week of the month, this shifts to Celebi.
All of these Pokemon are essential to unlocking content in the upcoming versions that can't be recieved otherwise, with the Legends giving access to Zoroark and Celebi for Zorua.