Centuries ago, Celtic people who lived in Nothern France, Great Britain and Ireland areas believe in spirits (both good & evil).
The Halloween story begins when these people spent a lot of time growing vegetables & food during spring and summer to, finally, recolect everything for the winter: when it was time to harvest all the food. Then, these people said "thanks" to the good spirits for your help.The harvest started when the days became dark & cold: November 1st.
That day became known as "All Saint's Day" or "All-hallowmas". The night before the festival day o celebration, October 31, was well-known as ""All-hallow's Eve" or "Hallowe'en".
"This is Halloween" by Tim Burton
(The Nightmare before Christmas intro)