In Rome, Pope Leo III placed a golden crown on Charlemagne's head during Christmas Mass, proclaiming him 'Emperor of the Romans.' The act, whether fully anticipated by Charlemagne or not, fused papal authority with Frankish military power and laid the groundwork for the Holy Roman Empire, a political entity that would shape European politics for a millennium.

Isaac Newton was born prematurely in the manor house of Woolsthorpe-by-Colsterworth, so small, by his mother's account, that he could have fit inside a quart mug. Under the Julian calendar still used in England, his birth fell on December 25, 1642; by the Gregorian calendar adopted elsewhere in Europe, the date was already January 4, 1643.

Amid a fierce nor'easter of sleet and snow, George Washington ferried roughly 2,400 soldiers across the Delaware River from Pennsylvania into New Jersey, using Durham boats to navigate ice floes in darkness. The daring maneuver set up a surprise dawn assault on Hessian forces at Trenton the following morning, a victory that revived a flagging Continental Army after a string of defeats.

From Brant Rock, Massachusetts, Canadian inventor Reginald Fessenden broadcast a short program of violin playing, a phonograph recording, and a Bible reading via amplitude-modulated radio waves. Ships' radio operators, accustomed only to Morse code dots and dashes, heard a human voice come through their headphones for the first time, an event some historians mark as the birth of AM radio broadcasting.

Though the most famous fraternization of the unofficial Christmas Truce occurred on Christmas Eve, informal ceasefires, joint burial parties, and even a few reported football matches continued into December 25 along stretches of the Western Front. Soldiers who days earlier had been trying to kill one another exchanged tobacco, buttons, and songs, an episode commanders on both sides moved quickly to stamp out in subsequent years.

After ten lunar orbits and a Christmas Eve television broadcast in which the crew read from the Book of Genesis, Apollo 8 astronauts Frank Borman, Jim Lovell, and William Anders splashed down in the Pacific Ocean and were recovered by the USS Yorktown, completing humanity's first journey to the Moon and back.

In a televised address, Mikhail Gorbachev announced his resignation as president of the USSR, effectively ending the Soviet Union's 74-year existence. Hours later, the red hammer-and-sickle flag over the Kremlin was lowered and replaced by the Russian tricolor, closing the Cold War era and setting fifteen newly independent states, including Russia and Ukraine, on separate paths.