Hi,here is a post on the history of Halloween.
The roots of Halloween can be traced back to a Celtic festival. Ancient Celts believed that on Samhain, or Halloween, the border between the Otherworld, the realm of the dead, and ours was thinner. Releasing spirits, both harmless and harmful, into this world. The Celts honored their ancestors and warded of harmful spirits. This was probably how costumes and masks came. The Celts probably used costumes and masks to pretend they were harmful spirits, protecting them self.
Samhain was also the time for preparing for the winter. The Celts slaughtered the livestock and had bonfires. They through the bones of the slaughtered livestock into the bonfires. Sometimes they made two bonfires and walked with their livestock between them to cleanse them selves.
That's it for now. My source was wikipeadia. Here a link!