On the 27th November, at the Council of Clermont, Pope Urban II preached what became the First Crusade - the various versions of the sermon preserved can be found here. In terms of military history, this was an important event, but because it's Thanksgiving I will gloss over it and simply point out one of the ironies of history.
The excuse for the Crusade was that Christians were being denied access to holy sites in Jerusalem by the Seljuk Turks that had conquered the region in 1076. But by the time the Christian Franks got to the Holy Land (they took Jerusalem in 1099) ... the Seljuk had been thrown out by the Fatimids, who had re-conquered the city in 1098. Since the Fatimids were fine with Christian pilgrims, umm, in theory the Crusade was a waste of time - but of course access was just an excuse.