Halloween is celebrated annually on October 31st. It is a tradition rooted in ancient Celtic New Year and end of summer celebraton of Samhain. People believed that on this day the worlds of living and dead merged together. In order to keep ghosts away they would put out their home lights, wear costumes and sacrifice animals and crops on huge bonfires. Over the years this festival was influenced by, and combined with, Roman traditions of similar connotations and later with Christian celebrations of All Saint's Day and All Souls Day, becoming first All Hollows Eve and later Halloween.
One of the most popular Hallowen traditions trick-or-treating is a modern version of an All Souls Day custom when the poor would beg for food and were given "soul cakes" in return for their prayers.
Jack-O'Lantern tradition comes from an Irish myth about Stingy Jack who tricked Devil twice and was denied entrance to Heaven for dealing with Devil. Devil would not allow him into Hell for tricking him, but sent him roaming the Earth with only a small light inside a carved out turnip.