As the title states you get a free xbox anniversary prop, just log onto xbox live or go to there website to download! Its basically a present that your avatar opens up and confetti and what not comes bursting out! its free so go get it!!
http://marketplace.xbox.com/en-GB/Product/Xbox-Anniversary-Prop/00001000-753d-c213-c7ac-9cfc5858081c?cid=SLink