The origin of Halloween can be traced back to the Celtic festival Samhain, on which the Celts observed the end of the ‘light half’ of the year.  Later, people changed to celebrate on the eve of All Saints (Hallows) Day, the Christian remembrance on 1 November - this is the derivation of the word ‘Halloween’.  Yet, vestiges of its pagan origin still remain in the present festivities, such as lighting lanterns and wearing costumes.

More interpretations and interesting customs of this ghostly-but-not-scary festival can be found in the following articles.