I know that Halloween/Samhain has always fallen on October 31. But wouldn't it make more sense to celebrate it on the day of the midpoint between the autumn equinox and winter solstice? (LMK if I'm missing anything.)
It would, if the point of the thing were to celebrate a midpoint between two events, thus making the holiday a moveable feast (a holiday whose date is malleable and dependent on the timing of other events). But that's not the point of the thing, nor the reason for the observance. Allow me to clap on my Witchstorian hat and explain.
Halloween is technically a Catholic holiday, the night before All Saints Day on November 1st, when all the saints in the calendar who do not have a dedicated day are honored. (And yes, there is a saint for every day of the year. Several for each, in fact.) It is also known as the Feast of All Saints, the Feast of All Hallows, All Hallows Day, and Hallowmas. (Honestly, Hallowmas kind of slaps and I wish it was still in common use.)
So it follows that the night before All Hallows Day is All Hallows Eve, which you may recall from some Halloween movies and media. With regional slang and linguistic shift, Hallows Eve became Hallows Evening -> Hallow Even -> Hallow E'en -> Hallowe'en -> Halloween. You can still see the "Hallowe'en" spelling in books and articles from the early 20th century. The apostrophe stuck around for quite a while.
It's interesting to note that November 2nd in the Catholic calendar is All Souls Day, when the faithful remember and pray for the souls of the dearly departed. Combined with All Hallows Eve and All Saints Day, it comprises a small three-day observance called Hallowtide, historically a time to remember and honor the dead.
So Halloween is datebound because of another holiday. But what about Samhain?
Victorian and Edwardian speculation about Ancient Celtic Pagan Practices notwithstanding (there are no written pre-Christian written records that cover it in depth), "Samhain" is the word for the month of November in Gaeilge (Irish Gaelic) and Gàidhlig (Scots Gaelic). The end of the month of October marked the end of the harvest season, so a festival was held to celebrate one more time before the long cold nights of winter fully set in. (October is Deireadh Fómhair in Gaeilge and Dàmhair in Gàidhlig, if you're curious.)
Technically speaking, Samhain is November 1st, not October 31st, but when your day ends at sunset, that means the night of the 31st is party time. The Wheel of Year generally marks Samhain as October 31st, but some models note that it goes from October 31st to November 1st. There are three other seasonal festivals in the Gaelic calendar that also occur at the beginning/end of a month - Imbolc, Bealtaine, and Lughnasa. And yes, these are still observed in the Isles today.
I hope this helps to clear things up! Enjoy the holidays! 😄🎃