They started celebrating because Immigrants from Ireland came to the United States and it became popular here.
When is it celeberated
It is celebrated on October 31st
Traditions
Some of their traditions are:
Kids and teenages go trick and treating which is when they dress up in colorful costumes. They go in their neighborhoods and beg for a treat or they will perfom a trick
People dress up in all kinds of costumes and go to parties
Also people decortate their houses to be scary
More Information
Here are some of the websites I used to gather this information
This website I used that had a lot of information on Halloween. bk2usa.com