Halloween was originally known as Samhain which marked the end of the harvest, the end of the "lighter half" of the year and beginning of the "darker half". It was traditionally celebrated over the course of several days and marked the beginning of the Celtic year. It has some elements of a festival of the dead. The Celts believed that the border between this world and the otherworld became thin on Samhain when some animals and plants were dying, and therefore the dead were able to reach back through the veil that separated them from the living. The Celtic custom of wearing costumes and masks was an attempt to copy the spirits or placate them.
To answer the question what year did Halloween start, it's important to recognize when the festival became associated with the day after known as All Saints Day. The holiday was first recognized by the name Halloween in the 16th century as a Scottish shortening of the fuller All Hallows Eve which is what the night before All Saints Day became to be known as.