According to Wikipedia:

Halloween (Also called Hallowe'en, All Hallows Eve, All Saints' Eve, Samhain, Spooky Day, Snap-Apple Night) is a tradition celebrated on the night of October 31, most notably by children dressing in costumes and going door-to-door collecting sweets, fruit, and other treats. It is celebrated in parts of the Western world, most commonly in the United States, Canada, the UK, Ireland, Puerto Rico, and with increasing popularity in Australia, New Zealand, as well as the Philippines. Halloween originated as a Pagan festival among the Celts of Ireland and Great Britain with Irish, Scots, Welsh and other immigrants transporting versions of the tradition to North America in the 19th century. Most other Western countries have embraced Halloween as a part of American pop culture in the late 20th century.

Read the complete Halloween entry on Wikipedia
Take a look at Happy Halloween! by detroitarts
Take a look at Happy Halloween by Detroit News Photo Blogger, Brandy Baker
Take a look at Happy Hallowe'en! by Lost Arts studio
Visit the Pumpkin Patch Pool on Flickr and try not to get lost!