Halloween marks the beginning of the "dark half" of the year. Although a few weeks have passed since the Autumnal Equinox, due to the seasonal temperature lag, the more natural division of the year into halves is a few weeks later. By now the Fall Foliage season is essentially over and the first Snow has either already fallen or soon will. Without doubt, Frost and Freeze have put and end to the garden for the year.
Of course the Holiday has been commercialized into an occasion to buy candy and costumes for young ones. But it's origins go back to a time when people believed that it was the day that the veil between the world of the living and the world of the dead was most easily parted, allowing members of one world to enter the other.
A scary thought, that ghosts and evil beings would be among us for an evening.