Black Christmas started on Dec 24, sunset in London, UK. It is the when the Grinches first attacked… and The Elfs came to Humanity’s rescue. Technically it continued until Epiphany (12th Day) when The Bomb was sent through the Wreath stopping The Grinch threat. (or so we think).
To understand all the context for this check out the FreeText Black Christmas.
or go to Black Christmas the Campaign. The trailer there makes the difference.