Halloween came from a religious holiday called All Souls Day. It was a day to honor the dead, but was eventually dumbed down (at least in America) into dressing up in random costumes for fun.
The History Channel website explains that a little bit:
"Even later, in A.D. 1000, the church would make November 2 All Souls' Day, a day to honor the dead. It was celebrated...with big bonfires, parades, and dressing up in costumes as saints, angels, and devils. Together, the three celebrations, the eve of All Saints', All Saints', and All Souls', were called Hallowmas."
Here's the link:
http://www.history.com/minisites/halloween/viewPage?pageId=713
They are not holidays in that, they are not days off from school or work. But they ARE holidays in that we celebrate something on the days (at least in my opinion.)