All Hallow's Eve comes each year the October 31. 
The Celts from Great Britain and from France celebrated a rite called Samhian. This recurrence, that marked the end of the summer represented the moment of the year in which 
the veil between the worlds  became thinner. 
The living persons could communicate with the dead people, and the dead persons returned on the earth.
In many cultures the autumn delimits a period of reflection . The Celts believed that magic beings were very strong in this period and that these creatures were often  hostile to the men, because they invaded the earth. So during this night the spirits wandered in the world to attract the human and trap them with their spells.

For the witches Samhain is one of the four Fundamental Festivity, 
or  Great Sabbath.  The not Christians believe
that Samhain is the most magic night of the year. It is at the opposite from
 Beltane (1 May) on the Wheel of the Year. 
It is the night of the  jack-o-lantern, and of the disguises. 
One night in which  dreadful histories of ghosts  are told in front of the fire. The correct time for the magic rites.

 We thank the site of the book of the shadows (in italian) where we found information on the Sabbath and on the history and Wiccan culture.