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Where did Halloween come from? The evil Druidic Holliday!  Haha, don't be so sure here's a few words from my own Grandmother WE call her Mami.  She Irish doesn't help much I know but she remembers a few things that might help you out... oh and I'm a college student so as usual I did some research.
A little not so well known fact:
Halloween- this is a Roman Catholic holiday!  the name means "All Hallows or All Saints' Day"
to the Roman Catholics this day is Nov.1.  It was created to honor all
the saints in heaven.  Nov. 1st was also called Halloweenmass, taken from Old
English, the word Hallow means sanctify.  This day was preceded by a vigil the
evening before on Oct. 31 known as "all Hallows' eve" or "Halloween".
Have you seen me before? ..... I've seen you....
But and However,~
Halloween is really of Druidic origion.  The customs we use are connected
with the day of the Keltic or Druidic New year.  The Night before the first day
of the new year, ghosts and demons of all kinds were said to run free all the
night.  So the people would leave out food and treats for the demons so they
would not harm their home.  The poor would send their children out dressed
as demons to collect the food.  There we have our costumes.  Also the Druids would
dress in honor of the demons and spirits, that might also give cause to the
costumes. 
The reason we have this Day is because the Druids (kelts) were conquered
by the Romans. And Well why not we use their callender also.

That's the Basics, Now you want to go deeper I can do that
Haveing a Kelt Grandmother really helps! Thanks Mami ^^
where are they? WHERE ARE THEY?!!!!
yes.. no you remember don't you?
From the Words of Mami~ Aye, these people here (americans) they do things
the way we did.  'Tis be a Scottish one too, nay that we didn't have them (irish)
'Tis before the christians came bringing their christ.  We had a great 'bration,
aye, the Druds (druids) pay to their Lord of the Dead, Saman they called him, but
is was nay on the 31st, 'tis be on the 1st of Nov. 
oh, aye, the Druds (Druids) they passed down their traditions by their
words, nay a part of it did they write.  I only  know what I 'twas told by me Papa
when I was a wee little thing.  OH the stories he'd tell me for "All Hallows".  But
only a few of us did so.  We'd know of the 'bration for years, after the missionaries came
from England and such things were forgotten and given up. 

~note from creator~
-mami, is my grandmother, and as much as she is correct, we must not believe
that our "halloween" came from Ireland, because it was never conquered by the
Romans, there's no recored of such a thing happening.  The ture origion of Halloween
to me is yet to be found, I believe it just all came together seeing how America is
the "melting pot". ~end note~
I shall take you back...
From the Words of Mami~
-'Tis be true there is such a connection of the Celtic, aye called him.. "Belenos"
they did.  They did some strange things in honor of that one..
~creator's note~ Belenos might have a connection to Baal (if you've read the bible
you know about Baal) but if might just be the words themselves.

Back to Mami~ from me father's words, here (america) when his father had come over
"all hallows" was nay seen, But after the potatos all died and they all came
from whince I Hail, the custom changed.  Now a day I see all the childern in their
costumes, with their Pumkin Giant heads, Pumkins (from an old story "The Pumkin Giant
email me if you want to know about it) oh, a sight is 'tis.
This "Trick o Treat" this an intresting thing, nay a child know what's 'tis been said
Pagans, what me father called them, they believed when the night the souls of the dead returned to
their origional homes (sounds to me like Los Dias de Los mueltos) you set food out for them, 'tis be the
"treat".  Nay did ye put any out the demons would "trick" you and there you'd have a spell on ye self or ye'd be
hurt terrible.