The Celts, Irish, Scotts, Welsh, Anglo-Saxon, and Brittans, had
a religion called Druidic Shamanism which is Religion of Nature
Worship. They believed the god of summer and winter, which is life
and death. For Halloween, they focused on Samhain, which is the God
of Winter and Harvest.
The festival of Samhain is on November 1st but Samhain's Eve is
on October 31st. And on October 31st the wall between the living
and the dead is at its thinnest where you could actually talk to
dead relatives (so they say). But, the spirits could also come into
our world and physically harm us on that day.
To be safe on Samhain, they would start bonfires and wear
disguises so the ghosts would stay away and think that they are
ghosts as well. They also put "actual" skulls on their houses so
the spirits will know not to mess with you. Some people didn't have
skulls so they carved turnips (they grew turnips there). That was
the early "Jack O' Lantern".
The Romans conquered them and decided to make a holiday too on
October 31st called Feralia, in which they celebrate Pomona which
is the Roman goddess of harvest. That is when the Romans invented
bobbing for apples, in which we use today.
People though it was bad celebrating death, so they created a
new "catholic" holiday when Christianity was legal because of the
Edict of Milan.
On October 31st, they would celebrate All Hallow's Eve
On November 1st, it would be All Hallow's Day (All Saint's Day)
in which they celebrate the saints.
On November 2nd, people celebrated All Soul's Day to pray for
deceased loved ones that died recently because they thought it
should be more for their loved ones also.
On that day, peasants would go around knocking on people's doors
asking for soul cakes, which is bread with crosses on the top in
exchange for them to prayer for their deceased loved ones in which
they would light a candle at church and pray all day.
When they went to America during the Great Potato Famine, people
got confused with the name "Hallow's" and it changed to "Halloween"
in which Irish kids would go around asking for soul cakes in
exchange for prayers. (The Irish kids went to the store to look for
turnips (skulls) but instead found huge orange things that they
found out were called Pumpkins and decided to use that instead).
People who didn't believe the holiday found out, and went around
saying give me stuff or they'll threaten them. Halloween turned out
to be a bad thing and actually banned in the 1900s until they were
legal again for kids to go around to people's doors saying: Trick
Or Treat.
Trick Or Treat, which people don't know, means give us food or
we will commit vandalism. People now, don't think of it as that,
but that is what it means when you go around knocking on people's
doors.