My theory is that Hell Night, or Devil's Night, exists for the same reason as
Halloween -- that is, a calendrical
glitch. At one time, goes the story, the
Irish civil year ended at the same time as the
grain harvest, and started up again on
All Saints' Day. This meant that there was an overlap of years in some years (when grain was harvested later) and a shortfall in others, when it was neither one year nor the next. Crimes committed during that time would be hard to prosecute in a
court of law, therefore it was a good time to engage in any
illegal or semi-legal activity, including petty
revenge,
fortune-telling,
witchcraft, and the like, while in the public mind, the very idea of time grew nebulous -- if it wasn't last year or next year, wouldn't it be no-time-at-all, and dead folks and the like able to walk around as if they were alive? (My thanks to
P.L. Travers, who pointed this out.) Most people just stayed home, with lights in the
windows, and anyone walking after dark tried hard to
disguise themselves, no matter what the reason.
FWIW, the tradition of shady tricks at night is older than the relatively recent one of "trick or treat", which was invented out of whole cloth by the Boy Scouts in the 1930's as a "safe and sane" alternative to the hijinks -- only later did it become associated with mourning rituals of the Ancient Greeks. Sigh.