Quick answer: Clear the game on Wayfarer, Pathbreaker, or Hellwalker to make 66 Days available. This mode uses Hellwalker difficulty and allows just 66 deaths.
What 66 Days mode is
66 Days is the fourth difficulty in Phantom Blade Zero. Its description reads “Same difficulty as ‘Hellwalker,’ but deaths are limited to 66 times. Unlocked after completing the game on any other difficulty.”
The rules for enemies match Hellwalker, which is the toughest of the three modes available from the outset; you can die no more than 66 times. For details on all four settings, see the difficulty modes guide; the Hellwalker guide explains how to prepare.
On the New Game screen, moving the highlight to 66 DAYS makes the backdrop red and displays its description along the bottom.
How to unlock 66 Days
In our run, before the ending, moving the Select Difficulty row onto 66 DAYS and applying it did not change the mode: the Game page still read WAYFARER afterwards.
In our run, after the Final Battle ending, the Select Difficulty page still listed 66 DAYS with the same description.
Once the ending is complete, Inanis at the Realm of Mirage makes New Cycle available, which is the game’s New Game Plus. Find her with the help of the New Game Plus guide.
Why it is called 66 Days
The number comes from the story. In the prologue, the Flesh Sculptor gives Soul the Tainted Heart, and the deal that sends him to Pangzhen ends with “Bring me that Heart, and I will make it yours. Then you’ll live beyond sixty-six days!”
In 66 Days mode, the story’s deadline serves as the death allowance for the entire run.
The Days Remaining counter
The days are counted on every difficulty, not only in 66 Days mode. On Pathbreaker, the Chapter 1 card, AGAINST THE FLOW, showed “Days Remaining:66” in the lower right corner.
In our run, the count fell to 65 after the first rest at a Crane Bell that played a memory.
In our run, it read 61 when Soul reached the Village of Lu, and 54 much later, in the snow.
In our run, deaths on Pathbreaker did not lower it. The death screen showed only DEAD under red calligraphy, with no day count, and the counter was unchanged after the respawn. The guide to what happens when you die covers the rest.
