I've seen some people posting that Halloween is supernatural. That's it is the day upon which Satan is strongest, or that the barriers between this world and the world of the dead are weakest.
So is that why Halloween is on October 31st? Because October 31st is a day with specific supernatural properties?
A brief history of how Halloween acquired its date
The Christian church has always sanctioned praying for the dead. The earliest evidence of this we have comes from inscriptions in catacombs from the the second century AD, and there is evidence (from the writings of Saint Augustine, 354 - 430 AD) of prayers for the dead being chanted as part of church services.
These later became formalised into the liturgy as The Office of the Dead, Requiem aeternam dona eis Domine: et lux perpetua luceat eis. (translation: " Eternal rest give unto them O Lord: and let perpetual light shine unto them. ") (source) (source) (source)
609AD - Pope Boniface IV created an annual celebration in the church calendar, to take place each May 13th, the Feast of All Holy Martyrs, to remember any martyrs who hadn't managed to be given an individual saint's day.
837AD - Pope Gregory IV renamed it to the Feast of All Saints, extended it to cover all saints, not just martyred ones, and moved the date to November 1st. He made the following day (November 2nd) be the Feast of All Souls (those about to become saints).
998AD - because the Catholic church didn't allow holding a feast and a vigil on the same day, Odilo, (an abbot at Cluny, in the Brittany region of France) started holding a vigil for the souls of people being cleaned of venial sins in purgatory the day before the two feasts.
During the following years, tradition built up around this in the surrounding countryside:
Breton families prayed by their beloveds' graves during the day, attended church for "black vespers" in the evening and in some parishes proceeded thence to the charnel house in the cemetery to pray by the bones of those not yet buried or for whom no room could be found in the cemetery. Here they sang hymns to call on all Christians to pray for the dead and, speaking for the dead, they asked prayers and more prayers.
Late in the evening in the country parishes, after supper was over, the housewives would spread a clean cloth on the table, set out pancakes, curds, and cider. And after the fire was banked and chairs set round the table for the returning loved ones, the family would recite the De Profundis (Psalm 129) again and go to bed.
It is only later, as this started to become a formalised celebration in its own right, and spread to other parts of Christendom, that a superstition arose that on thise particular three nights the souls of loved ones could return the Earth. During the night a townsman would go about the
streets ringing a bell to warn people that it was unwise to roam abroad at the time
of returning souls. (source) (source) (source)
Did the later celebrations (particularly in Scotland) become conflated with earlier pagan celebrations from the same season of the year? Yes. Did the pagan celebrations influence the picking of the specific date October 31st? No. When you read up on the history of Odilio, it becomes quite evident that there is nothing at all supernatural about that particular date and, indeed, it would have been held on November 2nd if not for inconvenient church regulations about vigils and feasts.