Halloween’s origins go back to the ancient Celtic festival Samhain (sow-in).
Celts, who lived 2,000 years ago, in what is now Ireland, believed
that on the night before New Year's Eve the barrier between the worlds
of living and the dead became blurred. On the night of October 31, they
celebrated Samhain. During this celebration, the Celtic people
wore costumes, (mostly animal heads and skins). They believed that
the ghosts of the dead came back to Earth, caused trouble and
damaged crops. They
also thought that these spirits made it easier for the Celtic priests to
tell the future. Life as a Celt was very difficult, so they needed these
"beliefs" to relax and be entertained.
A.D. 43, Romans took over most of Celtic territory. When they came to
their new territory, two of their customs combined with customary Celtic
festivity of Samhain.
the 800's, Christianity spread into Celtic lands. In the seventh
century, Pope Boniface IV nominated November 1, which is All Saints’
Day, as a time to honor saints and martyrs. It’s
widely believed that by creating these days the Pope was trying to
replace the Celtic festival of the dead with a related, but
church-sanctioned, holiday. The celebration was also called All-Hallows
and the night before, the night of Samhain, began to be called
All-Hallows Eve and, eventually would be called Halloween. Even later,
in A.D. 1000, the church would make November 2 All Souls' Day, which was
to be called a day to honor the dead. It was celebrated similarly to
Samhain, with big bonfires, parades, and dressing up in costumes as
saints, angels, and devils. Together, the three celebrations, the Eve of
All Saints', All Saints', and All Souls', were called Hallowmas.
This eventually turned in to Halloween.