VG247: Bungie and Microsoft just put out a press release detailing everything gamers can expect from the developer on Monday:
"The tradition of Bungie Day was born in part due to the significance the number 7 has played in all of Bungie's games over the years, including the "Halo" series (7 Halo rings, Master Chief is Spartan 117, etc.), and each year the developer makes a point to use this date to treat fans to something special – usually new content, a free download or other surprise."