The November 1st festival was called Samhain
(pronounced "sow-en"). Many people would parade in costumes made
from the skins and heads of animals. This festival would become the
first Halloween.
The invading Romans introduced their festivals and customs. One of
these was Pomona Day, named for their goddess of fruits and
gardens. It was also celebrated around the 1st of November. After
hundreds of years the Celtic's Samhain festival and the Roman Pomona Day
mixed becoming a fall holiday.
In the year 835 AD the Roman
Catholic Church made November 1 a church holiday to honor all the
saints, called All Saint's Day, or Hallowmas. Years later the Church
would make November 2nd a holy day called All Souls Day to
honor the dead. It was celebrated with big bonfires, parades, and
people dressing up as saints, angels and devils. Over the years all
these customs mixed making October 31 All Hallow Even, eventually
All Hallow's Eve, Hallowe'en, and then - Halloween.