All Hallow’s Eve
The modern version we call Halloween is an amalgam of early Catholicism and autumnal festivals celebrated around the world and dating back to forever. As the harvest ends and shadows grow longer, people have historically associated the change of seasons with dying and death.
November 1st, All Hallow’s Day (Dia de Todos os Santos, All Saint’s Day, Dia de los Muertas), is the yearly remembrance of our dearly departed. People hope/expect the souls they loved so well in life to visit. Graves are cleaned and decorated, remembrances and offerings are left, memories cried over during celebrations and feasting. The night before All Hallow’s Day is, of course, All Hallow’s Eve(ning) and has been shortened to Hallow’een.
Over the centuries (and through the richness of storytelling around warming fires) the idea of lost souls with no one to mourn them also being up and about on that day took over active imaginations. What would these lost souls (ghosts) want more than anything? To take over the body of someone living.
Jack o’lanterns were carved into faces and planted on gates and around doorsteps to fool evil spirits into entering the pumpkin instead of those sleeping inside the house. That eventually grew into disguising innocent children on that night, too, so evil spirits wouldn’t recognize them and take over their souls. That’s why they dress up as the wicked and scary, so they pass among the evil ones without being recognized as the innocent souls they are. The tradition of giving treats to kids in costumes at your door was a wink/nod at tricking the ‘evil spirits’ into leaving their homes.
Satanism celebrates and worships Satan and evil. It’s a whole different vibe and intent, which has absolutely nothing to do with Halloween.
If you are still curious, I highly recommend The Halloween Tree, by Ray Bradbury. It’s a glorious short novel that explores different autumn festivals and how they’ve become associated with Halloween.