Anytime from the start of the game, after unlocking at least six different weapons.
Follow my other guides here on GTN to unlock different weapons. You'll start with the Whip and Axe, but you should have at least unlocked the Clock Lancet, Peachone, Cross, Garlic, Fire Wand and Lightning Ring.
Every time you level up, select a new weapon when you can until you're carrying SIX in total. You'll unlock the EMPTY TOME for use in future runs.