Where Halloween actually comes from.

The Celtic root — Samhain. Around 2,000+ years ago, the Celts of Ireland, Scotland, and northern France marked Samhain (roughly October 31–November 1) as the boundary between the light half of the year and the dark half — the final harvest, when livestock came in from pasture and stores were prepared for winter. It was considered a liminal time when the boundary between the living world and the dead thinned, making it a night for honoring ancestors, but also one requiring protective measures — bonfires, disguises to confuse wandering spirits.

The Christian layer — All Hallows' Eve. In the 8th–9th centuries, the Christian church established November 1st as All Saints' Day (All Hallows), with October 31st becoming All Hallows' Eve — Halloween. Historians debate how directly this was a deliberate placement over Samhain versus a separate development that later merged with it, but the two became thoroughly intertwined in the British Isles over the following centuries. All Souls' Day (Nov. 2) added a specific day for praying for the dead, reinforcing the "veil is thin" association independent of its Celtic origins.

The immigration layer — becoming American Halloween. Halloween as most Americans know it took its modern shape largely through 19th-century Irish and Scottish immigration, which brought older harvest and divination customs (including turnip-carving, which became pumpkin-carving once jack-o'-lanterns met the New World's more abundant pumpkins) to the US, where they gradually merged with existing community harvest festivals and evolved into the trick-or-treating, costume-focused, largely secular holiday recognized today.

Where the practices come from

• Jack-o'-lanterns — from the Irish folktale of “Stingy Jack,” and turnip-carving traditions meant to ward off wandering spirits. Full story: A Short History of the Jack-o'-Lantern.
• Trick-or-treating — echoes of “souling” (going door to door for soul cakes in exchange for prayers for the dead) and “guising” (disguising oneself, tied to the same spirit-confusion logic as Samhain masks). Full story: The Real History of Trick-or-Treating.
• Costumes — directly descended from Samhain's disguise tradition, later folded into the general “dress as something else” custom of the holiday broadly.

Wondering how this all relates to the sabbat specifically? See Samhain vs. Halloween: What's the Actual Difference?