I don't see your point. The above context is the implication SF wasn't meant to be successful so nothing was planned out around the birthday besides I cool date to pick. So first I ponder when Harry's birthday actually came up, point out other birthday's have the same kinda consideration around their picking, like Thomas on Valentine's day. An point out the timeline between when he actually wrote the first draft and published it had a big overaching story brainstorming session. If your having trouble following along, feel free to ask.
I
suspect (but do not know for a fact) that Jim Butcher's initial ideas of HBCD & DF included a vague "Chosen One" concept. That he picked "Halloween" as "a cool date."
Jim was already a RPG'er, a LARP'er. Me may have had pagan friends... and even if not, the ideas of "Samhain" and the "thinning of veil between mortal and otherworld" had definitely seeped into geek/gamer culture by then!
I don't know if he had the "Starborn" notion, or any "the stars [specifically] are right." He
could have been a "Chosen One" because of his bloodline, or because of some Sacred Chrism applied at birth, or any of several other ways; AFAIK, nobody has ever sought answers from Jim as to why he picked a "Starborn" / "conjunction" origin-story for his Chosen One.
(It could as easily have been something about that liminal, All-Hallows-Eve state and just a 666-year accumulation of supernatural potential that settles on one "lucky" mortal every 2/3-millenium...)
I'm just going to assert that Jim didn't actually research that there was any "meaningful" or "significant" conjunction (in the conventional (i.e. astronomical or (more-likely) astrological senses of meaningful or significant) that happens on Halloween every 666 years, and no other times.
There is no
there there.
He did a bunch of brainstorming, but I don't think he ever went back to these origin-story questions (or maybe he
did... but could not discover such an astro<whatever> event, and he was stuck with some of his 1st-draft "early-episode wierdness").