Hallowe'en is of non-Christian origins. The Celtic peoples celebrated the festival of Samhain at the beginning of the dark half of the year, about November 1.
The Church retained the celebration, but gave it a Christian significance by changing the focus to honour all the saints, both known and unknown. This became known as All Saints' (or All Hallows') Day. The day following became All Souls Day, a day to pray for the souls all the dead.
In the Celtic tradition of beginning the festivities at sundown of the preceeding day continued in the celebration of All Hallows' Eve, what we now know as Hallowe'en.
These early people saw winter as a time of dying away of the sun and the earth and crops, a time of food shortages and danger. They believed that at that time the space between the living and the dead, as well as between the seasons, was particularly fragile, making it possible for the spirits to cross over into the world of the living. Bonfires were lit and fortune-telling was a favourite activity.
Masking was also part of the medieval tradition, as it remains today. People were very superstitious, believing in the power of demons and ghosts. The Church was concerned that dressing up as these figures would give the demons and ghosts extra power. The actual result was that by making them the figures of fun and ridicule, demons and ghosts began to lose their strength over the lives of the people.