Happy Birthday, Lauren!
To access the easter egg, simply change the Xbox 360's date to Christmas day (i.e. 25th December or 12/25/2019). Then proceed to loading any campaign level and press both thumbsticks down. The loading screen will start up but from a different angle, showing a scrolling message reading, "Happy Birthday, Lauren!"
- Bungie.net: Mail Sack 13 (Adrian Perez: "Showing my wife the Easter egg I put in the halo 3 loading screen for her - the one that nobody has found yet.")