I doubt we'll ever know... but here's some ones that I've found so far...
This author's article speculates that it's an Easter egg that plays on several ideas:
• Halloween is the carnival time, a ritual day when one can pretend to be someone or something else, whether a comic book superhero like Batman or Superman, or another eternally popular choice for Halloween, a Ghost — a spirit, much like Satoshi, that is neither dead nor alive. • Halloween could be a reference to 'Hal' - Hal Finney
This author lists the possible reasons that others have speculated as being:
• Halloween originated as a celebration of death and renewal. The Celts celebrated the start of their New Year on November 1, so Halloween (or ‘Samhain’ as they would’ve called it) is equivalent to a New Year’s Eve party in those days. • Weese also draws parallels between Martin Luther and Satoshi Nakamoto, who both announced their ideas on October 31 to undermine the power of a central authority. While Luther undermined the Vatican in 1517, Satoshi’s innovation attempts to reduce the power of central banks. • Maybe it’s just a coincidence, but October 31 also marks the day that the National Security Agency’s (NSA) proposal for an implementation of anonymous electronic cash was received by Fried Frank LLC (a law firm focusing on antitrust and competition among other things) back in 1996.
Finally, you might be asking how was Satoshi's whitepaper received by his fellow cypherpunks on Halloween? Well, surprisingly, it didn't go as planned.
This author claims that Satoshi's whitepaper 'almost sunk without a trace' - claiming that Satoshi was forced to repost it on November 3.
However, for all we know the cypherpunx could have just been out trick or treating and just didn't see it.
I'm sure there's more crazy hypotheses out there though.
I think my favorite is the Martin Luther one.
One idea is that it's when the dead are meant rise up from our graves.
Well, us folks have all come alive now 👻