Valhalla Wall Easter Egg

Like Zanzibar in Halo 2, Valhalla has different messages located on the Forerunner Wall in Halo 3.

These can be accesed simply by changing the date on your Xbox 360.
 * 1/1: Make Love Not War
 * 10/31: A grave stone that says RIP
 * 7/7: Seventh Column Symbol
 * 7/4: BBQ at Bob's House
 * 11/11: Killroy Was Here
 * 5/5: Today Was Live!
 * 12/25: A diamond with an X through it
 * 4/22: A skull with a recycling sign through it