December 31, 406 CE
The day mentioned in the title is one of the landmarks in history. Although the Roman Empire was showing already significant signs of decay in its economy, culture and military strength, the day represents one of the triggers for the final demise of the Western side. That day, the Vandals, together with other barbarian tribes, crossed the frozen Rhine, which formed the natural boundary of the Roman Empire in the northeast, near Mainz. The Rhine frontier had been denuded of its best troops by Stilicho to fight Alaric and his Visigoths in 401-2 CE, and Radagasius and the Vandals, Quadi and Marcomanni in 405-6 CE. The formidable natural barrier of the Rhine backed by the few remaining forces left behind by Stilicho were enough to hold the Vandals in check until the severe winter of 406-7, when the Rhine froze. For the next three years they ravaged Galia (modern France), before moving south in 409 CE into Hispania (modern Spain) and Africa (modern northern Africa) in 429 CE.
The disaster at Gaul led to the execution of Stilicho and the subsequent mutiny of some 30.000 Germanic troops that defended the Eastern border of the Western Empire. This changed the balance of power in Italy in favor of the Visigoths who finally captured and sacked Rome in 410 CE.