Zombie Apocalypse
12 Achievements
5-7h
PS3
Anniversary of the Dead
Complete the Final Day, in either Single or Multiplayer, and finish the game
22.8%
How to unlock the Anniversary of the Dead achievement in Zombie Apocalypse - Definitive Guide
PS3Tkira
The final day is the 55th day. You just need to get to the 55th day, even if you do have to continue. If you don't continue, than your points just don't go towards the Leaderboards, but you are able to still get this trophy. (Thanks to SneakyWhite for this information)
NOTE: If you have a friend who has gotten to day 55, he can start the game on day 55 and invite you. After finishing that, you will get the trophy, even if you haven't done any of the other days.