The Greeting Card Association has released the following brief history of Halloween. They also say that today approximately 35 million Halloween cards are sold each year in the United States.
Halloween is an ancient festival that evolved from a combination of ancient
religious beliefs and the harvest celebrations of the Celts and their Roman
conquerors.
Many historians believe Halloween can be traced back to the ancient Celts,
whose New Year began on November 1. At the end of October, they celebrated
Samhain, an end-of-harvest festival that marked the beginning of the “dark” or
fallow half of their agrarian year.
This distinctive division between dark and light was also thought to be a time
the spirits of the dead could breach the world of the living. This led people to don masks or costumes as a way to disguise themselves from these spirits. Sometimes townspeople would parade in their disguises to the outskirts of their villages to lure the spirits away from their homes.
As the Romans conquered the Celtic world, they combined their own fall
harvest festival and celebration for the dead with the existing Samhain practices, and the unusual fall celebration continued to evolve.
During the eighth century, Pope Gregory III moved the All Hallows church
festival to the first of November. The night before, October 31, became known as All Hallow's E'en or "Halloween." In modern times, Halloween has always been
celebrated on October 31.
In the United States, Halloween was not widely observed until the 1840s,
when Irish Catholics fleeing from the potato famine brought their Halloween customs to America. Halloween decorations and cards began to appear in America around 1910, although most of them were imported from Germany. When World War I
broke out, imports were curtailed and American companies began to fill the void.
Today approximately 35 million Halloween cards are sold each year in the United States.
The 
