Edit - The offline method is much less reliable than it used to be. It does still work, but it may be much trickier to register.

Make sure you stare at the sign for a good few minutes and look at it from different angels. Also try shooting it.

To get this one you need to viewsigils on the Halo 3 map. Sigils appear on special days of the year. You can do this in forge, custom games or even in matchmaking. You can only obtain ONE sigil per special day. Change the date on your console to get a second one, or wait for the next special day to come. The sigil appears only for that day, so it will not be there the day after. The sigil can be found on the far wall located behind the turret. (Video is below)There are two ways you can get the achievement.- Only works with discIf you intend to use the offline method, it can be a bit annoying, but, it DOES work. You will not receive the achievement until you reconnect to Xbox Live and you will not receive a notification for it. It will also not appear in your achievements for a couple of minutes.How to do the offline method:1. Disconnect and forget your network.2. Restart your console.3. Set the date to one of the required days.4. Go and view the sigil.5. Reconnect6. The sigil should register when you go back onlineIn addition, many users have noted that this achievement is glitchy.Simply wait for each day to come and view the sigil.(Taken from Halo wiki)- January 1 (New Year's Day): Make Love Not War- April 22 (Earth Day): A skull with a recycling sign.- May 5 (Cinco De Mayo): Toady Was Live!- July 4: BBQ at Bob's Place (this is a reference to Marathon in which you kill BOBs or BOB-B-Qing).- July 7 (Bungie Day): Seventh Column Symbol.-October 31 (Halloween): A grave stone that has the initials "R.I.P.", meaning "Rest In Peace".-November 11 (Veteran's Day): Kilroy Was Here.-December 25 (Christmas): A Christmas light with an X on it.An example of a sigil can be seen here:Thanks to Maka91 for confirmation