Elder Signs - Similar to the beach sign on Zanzibar in Halo 2, this easter egg requires playing Valhalla in Halo 3 on different days. When done, different signs/sigils will appear in white paint on the grey/silver wall on the side of the map. The dates are as follows: Jan 1st, Apr 22nd, May 5th, July 4th, July 7th, Oct 31st, Nov 11th, and Dec 25th. One of these is launch day (Nov 11th) so make sure you load that up asap.
A video just showing the location (along with two other achievements), and the dates to trigger the various graffiti images are Jan 1st, Apr 22nd, May 5th, July 4th, July 7th, Oct 31st, Nov 11th, and Dec 25th.