Answer:
The main idea concerns the origins of Halloween.
Step-by-step explanation:
In the passage itself, we can say the main idea is expressed by the sentence "Halloween is believed to have come from an ancient Celtic festival dating back some 2,000 years." The purpose of the passage is to explain the origins of Halloween by presenting readers with facts about where it began and how and why it was celebrated. It shows how our "candy fueled celebration" came from an ancient tradition.