A Day To Remember
To unlock this achievement you must load up Halo: 3 on the 25th of any month, quite simple.
If you want to unlock it on any other date you can do so offline. Now this doesn't just mean you disconnect yourself and do it, you must first follow a few steps.
First going into your settings by hitting the button towards the middle of your controller with 3 lines across it (otherwise known as the start button).
Second go into Network.
Third "Go offline" and "forget wireless network"
Then you are going to need to hard turn off your Xbox by holding the power button for 10 seconds.
Once you turn it back on go back into settings, then system, to the way right you will be able to click on your date and change it to any day you want. For this achievement you change it to the 25th.
Load the game up and start Halo: 3, quit out once it loads up, reconnect yourself back to the internet and it will unlock for you.