User blog comment:SlyGentleman/Zombie Redemption DLC/@comment-3129197-20101023234225

The zombie outbreak will happen around the time that John is building his ranch. (Right after he got Jack and Abigail back) Also when you click "Singleplayer" at the main menu there will be an option where you can choose either "Red Dead Redemption" and "Undead Nightmare." You may already know this.