At first I thought it was the day after everything comes to a head, the day, in fact, at which (for magical reasons) he ceases to be able to save the day. (Nothing like a little pressure to make it nice to figure out what to do.)
Then I thought -- on the day? Nothing like a nice liminal period of transition from one state to the other, since there isn't a moment of birth.
But, of course, I figure out what they need to do to get there, and how long it will take, and then I have his birthday. And can have the characters sighing with relief. (Because if he doesn't do it, the next kids in line are four years younger than he is.)