Why was Halloween declared a holiday?

Answer 1

It was actually meant to scare away ghosts and phantoms who would rise from Hell on October 31st each year. It became tradition and as those beliefs passed it became a holiday.

Answer 2

Halloween has its roots in ancient Celtic harvest festivals and later Christian traditions. It was originally celebrated as a way to ward off evil spirits. In the 19th century, Irish and Scottish immigrants brought the tradition to the United States, which eventually led to Halloween becoming a popular holiday in America.