#1.) Trick-or-treating. It comes from an old tradition where poor children in England and Ireland would go door-to-door praying for the dead on All Saints' Day.
--After World War II, kids in the U.S. started doing the door-to-door walk, and people started giving them candy . . . which led to trick-or-treating.
#2.) Jack-o-lanterns. This comes from an Irish legend about a farmer named Jack who would play tricks on the devil AND God. That got him banned from heaven and hell, so he roamed the world as a flame inside carved vegetables.
--Jack-o-lanterns became part of Irish superstition too. People would carve turnips and put them outside their houses to scare away evil spirits. Americans used pumpkins instead because there were more of them and they were easier to carve.
#3.) Haunted houses. These don't come from any tradition . . . except the tradition of people finding a way to make money off holidays.
--In the 1950s, Junior Chamber International clubs realized they could make money off haunted houses that played off the Halloween spirit, so they did . . . and everyone else followed their lead.
#4.) Witches, ghosts, black cats, etc.
--You can thank a combination of the European interest in witches that was brought to America combined with the Native American beliefs in evil spirits. Oh, and you can thank the Druids for the black cat superstition, as they were convinced that black cats were humans gone bad.
#5) Costumes. Ready for more commercialism?
--A small part of the original Samhain festival was to either hide from or appease evil spirits, the rise of costumes as a major part of Halloween actually took off in the US as the consumerism of Halloween entered the fray, with reports of mass-produced costumes dating back to the 1930s, coinciding with the rise of trick-or-treating.
#6) Halloween itself traces back to the Celtic festival of Samhain. On Oct. 31, the worlds of the living and dead overlapped before starting the new year on Nov. 1, when All Souls Day is observed.
--The 31st became All Hallows Eve, a time when the ghosts of the dead could return to destroy the harvest that was stored leading into winter (because dead people are angry bitter?).