The term Celts is a misused term as the term " Celt " was not used at the time of the Ancient or Early People in the British Isles. The people were not "Celts " ; they were Gaels, Britons and Gauls. Celt comes from the Greek word Keltoi which means barbarian and was used from 17th and 18th Centuries to classify these people who lived a certain life style. The Celts.
Many of the ways we celebrate Halloween here and in America come from Gaels and Britons. Even then though, the festival was not called Halloween. It was known as Samhain( pronounced Sow -win ) Christians took the festival over and Christianised it to Halloween, All Hallows Eve.
Millions of Irish/ Britons who emigrated to America took these traditions with them.