Halloween is a holiday with roots dating back to ancient Celtic festival of Samhain, so there really isn’t a date that anyone can point to for when it started being celebrated. Samhain is still celebrated and recognizes the end of the harvest season and beginning of winter (the dark part of the year). The Celts believed at this critical time of the year, the line between the living and dead was blurred so offerings of food and drink were left outside for souls of the dead who may come to visit.
In the 16th century, mumming and disguises began playing a role in the festival of Samhain as people dressed up as departed souls and went door to door asking for offerings of food and wine. It was believed that by impersonating the departed souls, those dressing up would be protected from those spirits.
In the 19th century in the U.S. a movement began to turn from the darker elements of the holiday to a more community oriented celebration. And as we moved into the 20th century, the holiday became more focused on children with festive costumes and games like bobbing for apples. Today, Halloween is second only to Christmas in its commercial value to retailers.
Want to know more about the origins of Halloween? Check out these links: