Halloween, primarily celebrated in English-speaking countries, is a bizarre mixture of traditions from different periods of history. Over the last couple of decades, marketers have attempted to spread this Anglo-Saxon tradition to other countries, with varying degrees of success. When we lived in Europe, there was an increasing interest in certain aspects of Halloween, but is has never caught on to the extent to which it is celebrated in the US.
No one knows for sure when the tradition began, but most historians agree it traces its origins to a Celtic celebration called Samhain, whose names roughly means 'summer's end.' It was believed that, at this time of year in particular, there was a thin line between this world and the "otherworld," allowing different kinds of spirits to come and go. Hence the necessity to disguise oneself as a harmful spirit in order to ward off harm.
What is ironic is the origin of the name Halloween -- originally All Hallows E'en -- as in the eve of All Hallows, or All Saints Day. All Saints Day has been celebrated on Nov. 1 by the Catholic church, and more recently, other denominations as well, since the 7th century. This is traditionally a day in which the saints -- known and unknown -- are remembered.
Another great example of the strange juxtaposition in our culture of Christian and pagan elements. On one day, inordinate attention is given to the dark side, the Underworld -- and the very next day the saints are honored. The term "melting pot" could not be more à propos.