According to the Lotharingian computists, on this date the world was going to end. They believed they had found evidence in the Bible that a conjunction of certain feast days prefigured the end times. Supposedly, it was on this day that Adam was created, Isaac was sacrificed, the Red Sea was parted, Jesus was conceived, and Jesus was crucified. Therefore, it naturally followed that the End must occur on this day!
The Lotharingian computists were just one of a wide scattering of millennial cults springing up in advance of that first Millennium. The abbot of Saint-Benoit of Fleury-sur-Loire sent a letter to his king complaining about the Lotharingians:
For a rumour had filled almost the entire world that when the Annunciation fell on Good Friday, without any question, it would be the End of the World.
The millennial panic endured for at least 30 years after the fateful date had come and gone, with some adjustment made to allow 1,000 years after the crucifixion, rather than the nativity.