And no, it’s not bungie jumping day. Today apparently is Bungie (the people who made Halo) day, according to Microsoft. Today and today only you can download a Bungie Day 2007 Picture Pack and a Theme for free.
After today, they will disappear forever and you won’t be able to get them again (unless trading it to a friend somehow works).
Go check them out!
To find the free content, go to the marketplace and Select Games<New Arrivals<Halo 3.