Halo 3 Easter Eggs

On the map "Valhalla" on Forge on one of the following dates go to the big grey wall on the right to the red base and go behind the big brown rock next to the tree and you will see different things on different dates.

Dates:
1st January
22nd April
5th... more

Go into the campaign lobby and go onto the select difficulty. Go down to easy and look at the picture. It works on any difficulty, but it's easier to see on easy, and on the campaign shield is the Marathon symbol.