10/29/05 Halloween was celebrated by the Celts as "Samhain" ("Summer's End"),
before Christianity changed it into "Hallowmas" or "All Saints Day".
For the Celts, it marks one of the two most important periods of the year,
the other being Beltane, celebrated on May 1.
Beltane is the Light, and Samhain the Dark, the two great seasons making up the Celtic year.
At Samhain, the Celts celebrated the end of the year, and the beginning of a new cycle.
I found this nice description on-line:

"Some believe that Samhain was the more important festival, marking the beginning
of a whole new cycle, just as the Celtic day began at night.
For it was understood that in dark silence comes whisperings of new beginnings,
the stirring of the seed below the ground.
Whereas Beltane welcomes in the summer with joyous celebrations at dawn.."

At Samhain (as at Beltane), the mundane laws of time and space are suspended,
to allow for closer contact between the worlds..
Old Celtic lores recount of the Spirits of the Deads returning to visit us,
which is why lanterns were placed outside the door, to aid them in finding their way.