Nightmare Night and it's the Autumnal Equinox- when day and night are equal. The Winter Solstice is the 22nd of December this year IIRC and is the longest night of the year.
Though, really, the Winter Solstice would be a more logical place for a celebration of the night --Hallowe'en started out as a harvest festival (combined with a remembrance for the dead). The Equinox was the point by which all the crops needed to be in (because the temperature's dropping). The local Hallowe'en analogue would be somewhere between "Applebuck Season" and "Fall Weather Friends."
There was a documentary on it's origins last year on History channel, so I can't remember all the details, just that it originated from someone who liked to cause pranks and tricks, then was caught and executed, and to mock him, they dressed-up in his mask every year.
The change of date may have been something to do with St. Patrick's day, but I doubt it. I just know something was changed for religious reasons.
Maybe they'll have it on again this year, as the wiki only has references believing to link it to Irish, Scottish, Welsh, etc lore.
Actually, part of that is true, but it was combined from many different cultures and traditions and just sort of blended together over the centuries. I don't think the traditional masks even came into play until the 1920s. Mostly just the harvest feast combined with keeping evil spirits at bay and what not. It comes from so many different cultures and religions it is hard to pin down the actual source that started it all.