Anyone know how to unlock the 14th Day?

Fool.

 

Beyond Thirteen
Complete enough subquests (approximately 40) and all the main quests to reach Day 14. Day 14 will appear on Day 13, and not before. In other words, if you meet the conditions for this achievement (and for unlocking Day 14), on Day 11 you will read "Two Days Till the End" (as if you can only reach Day 13), on Day 12 you will read "One Day Till the End" (again, as if you would only reach Day 13), but then on Day 13 another Day will be added, allowing you to reach Day 14.

 

http://www.trueachievements.com/walkthroughpage.aspx?pageid=5949

Do a bunch of side quests on top of the story quests

Fool.

 

Beyond Thirteen

Complete enough subquests (approximately 40) and all the main quests to reach Day 14. Day 14 will appear on Day 13, and not before. In other words, if you meet the conditions for this achievement (and for unlocking Day 14), on Day 11 you will read "Two Days Till the End" (as if you can only reach Day 13), on Day 12 you will read "One Day Till the End" (again, as if you would only reach Day 13), but then on Day 13 another Day will be added, allowing you to reach Day 14.

 

http://www.trueachievements.com/walkthroughpage.aspx?pageid=5949

 

Do I need 40 by the end of Day 12?

I'm just at day 2 haven't played as much as i hoped

Do I need 40 by the end of Day 12?

Fool. No idea.