The central idea of the article "Halloween Owes Its Tricks and Treats to the Ancient Celtic New Year's Eve" is that Halloween has its roots in the ancient Celtic celebration of Samhain, which marked the end of the harvest season and the beginning of winter. The article mentions that Samhain was a time when people believed the boundary between the living and the dead was blurred, and they would light bonfires and wear costumes to ward off evil spirits. Additionally, the article explains that the tradition of trick-or-treating has its origins in the Celtic practice of leaving food and treats outside their doors to appease the roaming spirits. These details highlight the connection between Halloween and its ancient Celtic origins, emphasizing the historical and cultural significance of the holiday.