Along with the mistaken belief that medieval people thought the world was flat, one of the most persistent myths about the Middle Ages is that everyone thought the world was going to end in the year 1000. This is, in fact, untrue.
For one thing, most people had no idea what year it was. Without calendars, newspapers, cell phones that display the date, or the like, they would not have had any easy way of telling the year. When the year 2000 came along there was a quite real possibility of messed-up computers on January 1, as everyone heard about in great detail, but there was nothing similar a millennium earlier.
For that matter, there was not even general agreement then on when the new year began. Some places began the new year on January 1, which was the Roman norm. Others began it on Easter. This of course could create problems: if Easter was early one year and late the next, a year might have two months of April, one at the beginning and a partial one at the end. Others began the new year with the supposed date of the Incarnation, that is the conception of Jesus, March 25 of the previous year. So different regions, even different cities, could easily be a year apart in what year it was.
And of course, even for those who tried to keep track of the year, without outside verification it was very easy to slip up (remember, they were using Roman numerals, always confusing).
The millennial event which medieval people did celebrate was 1033, the thousandth year after the Crucifixion, when, as one chronicler put it, "France was covered with a white mantle of churches." No one expected the world to end--rather, they anticipated a spiritual awakening. (The image above is of Tournus, an eleventh-century church in southern Burgundy, an example of what the chronicler meant.)
However, some nineteenth-century scholars read these accounts of 1033 and misinterpreted them thoroughly. There was not indeed any suggestion, either by people at the time or by historians, of the "terrors of the year 1000" until the nineteenth century.
These nineteenth-century scholars took as "proof" some preambles to charters, "We who live in the final days…" There are two main problems with this "proof." First, these charters, which are only found in one or two places, all date to about 950, two generations too early to suggest fear of the year 1000. Secondly, and even more importantly, the scribes who wrote out these charters were using an old "formulary book," a handbook on what charters were supposed to look like. This formulary book dates from the seventh century. So the apocalyptic charter language which some thought "showed" that people feared the end in the year 1000 actually showed that at least one person feared it three centuries earlier.
In fact, there have been repeated predictions over the centuries that the world was about to end. There certainly were charismatic preachers in the Middle Ages who announced the Second Coming was right around the corner, just not in the year 1000--and then as now, most people ignored them.
So far, the world has been very stubborn and persistent. In the Middle Ages, as now, the response to a failure of apocalyptic prophecy was generally to slip quietly home and pretend one never believed it anyway. Other possibilities included saying the end was still coming, but somehow the date had been miscalculated; here the difficulty was getting anyone to believe you next time. More imaginatively, leaders could announce that the piety and prayers of their little apocalyptic group had "saved the world" from destruction, or, even better, say that the world really had ended, but everyone else was too stupid to notice.
The one year that there really was a belief in the imminent end was 1260, but that's a topic for another day.