Like so many other good traditions, Halloween started in Ireland.
The Department of Foreign Affairs has put together an explainer video on the origins of the holiday.
It started with the ancient Celts over 1,000 years ago, who called it Samhain.
It was seen as a celebration of the harvest and preparation for the winter season.
The Celts believed that on Samhain, the souls of the dead would visit their homes - and that those who had died in the past year would travel to the next world.
People would also wear masks to disguise themselves from the visiting spirits.
Large bonfires were also lit in an attempt to ward off the evil spirits that visited on the night.
After the communal bonfires, people would hollow out a turnip and carry an ember of the bonfire on their journey home.
While the Irish also forecast future happenings at Halloween, by baking barm brack and roasting nuts to predict marriages.
The tradition of trick-or-treating comes from poor people begging for food, called 'soul cakes'.
When emigrants went to America in the 19th century, they took Samhain - or Halloween - with them.
But the tradition of carving turnips was changed to pumpkins, because turnips were so hard to find.
<blockquote class="twitter-video" data-lang="en"><p lang="en" dir="ltr">Happy Hallowe'en! Ireland is <a href="https://twitter.com/hashtag/homeofhalloween?src=hash&ref_src=twsrc%5Etfw">#homeofhalloween</a> - most of today's <a href="https://twitter.com/hashtag/halloween?src=hash&ref_src=twsrc%5Etfw">#halloween</a> traditions began with Samhain & went abroad with the <a href="https://twitter.com/GlobalIrish?ref_src=twsrc%5Etfw">@GlobalIrish</a> <a href="https://t.co/xSVX0M7PAJ">pic.twitter.com/xSVX0M7PAJ</a></p>— IrishForeignMinistry (@dfatirl) <a href="https://twitter.com/dfatirl/status/925308064105947136?ref_src=twsrc%5Etfw">October 31, 2017</a></blockquote>
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