It is just as relevant as December 25th being chosen for Christmas. It was agreed upon years ago and has become tradition.
If you're looking for some official pronouncement, here -- I'll make it:
Be it known herewith, henceforth and forever that the official start of the Christmas season begins on said date of 13 November and the countdown to such is led by the noble, righteous, and incurably merry Crown. He, and his minions, are hereby certified, notorized, authorized, dignified and hopefully not euthanized within these precincts to celebrate said season with all manner of mirth, witty reparte, wry observation, good humor and poignant repose.
This I declare by the authority vested in me as the domain proprietor of the only surviving Christmas portal dating back to the 20th Century and the owner of the largest, brightest, merriest, cheeriest and doggone Christmasiest website in the world.
So let it be written, so let it be done.
Hallelujah, holy sh*t -- where's the tylenol?