Halloween is celebrated on the night of October 31.  The word "Halloween" is a shortening of All Hallows' Evening or All Hallows' Eve.

Halloween was an ancient Celtic festival, known as Samhain (pronounced "sah-win"). 
This festival celebrated the end of the harvest season. People believed that on October 31, the worlds of the living and the dead superimposed, and the deceased could come back to life and cause destruction or sickness.

Masks and costumes were worn on that night, to try to mimic the evil spirits or appease them. 

In the late twentieth century, many countries embraced the holiday including Ireland, Scoland, the United States, Canada, the United Kingdom, Australia and New Zealand.

Nowadays,Halloween has evolved. The holiday is especially popular in the United States, the United Kingdom, Ireland and Canada. Traditional activities include first of all children's "trick-or-treat" (children go from house to house in costumes, asking for treats -sweets and candies- with the question "a trick or a treat?" If the homeowner doesn't give a "treat", the kids will play a "trick" on him). In neighborhoods with children, adults buy treats in preparation. There are also bonfires, costume parties, visiting "haunted houses" and carving jack-o-lanterns.