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.
 * January 1st (New Year's Day): Make Love Not War
 * October 31st (Halloween): A grave stone that says RIP
 * July 7th (Bungie Day): Seventh Column Symbol
 * July 4th: BBQ at Bob's House
 * November 11th: Killroy Was Here
 * May 5th: Toady Was Live!
 * December 25th (Christmas): A diamond with an X through it
 * April 22nd: A skull with a recycling sign