Did people do anything special to celebrate the year 1000 A.D.?

Only a small portion of the world counted their years from the birth of Christ. The principle source for stories of panic and anticipating Armageddon is from a French monk by the name of Ralph Glaber who had a section called “On the First Millennium” in his book Miracles de Saint-Benoit. Glaber was regarded as a troublemaker during his lifetime and was expelled from several monasteries. Subsequent writers made the situation worse by exaggerating his descriptions of the times. What Glaber actual wrote can be read on at www.fordham.edu/halsall/source/glaber-1000.html.

In general, there were millennial cults with doomsday predictions and everything like that in the past, yes. The classic source on this (which I read so long ago that my memory should not be relied on) is Norman Cohn, The Pursuit of the Millennium: Revolutionary Millenarians and Mystical Anarchists of the Middle Ages. One of Cohn's big points is that this sort of thing is actually extremely common whenever we have years or dates that have any kind of "resonance" for people (1666 was another big one). As for 999AD specifically, I'm not sure — it seems like it would in part depend on whether the general public was aware of what year it was in the AD system.

One thing we know about the year 1000AD is that iceland, a key seaport for shipping, voted to become a Christian nation, and therefore a Christian port (which would attract much new business) that year. Much of this is very interestingly referenced in "Njal's Saga", one of the most fun and interesting major sagas. Now, this doesn't exactly answer your question, as to whether or not elsewhere there existed cults or fanatical groups claiming that 'end times were nigh'. What it does tell us is that it was becoming financially untenable not to be a Christian port, and that not only was there a 'business as usual' attitude, but that business was expanding and becoming a more important part of the previously isolated area of Iceland. Edit: Also of note that the fairly pragmatic population changed it's religion out of financial concerns, again documented in that and other sagas, which discuss some of the debate, but what isn't present in the debate is an end-of-times argument. Interestingly enough, a course I took on Icelandic sagas used that and several others sagas to discuss the question of whether or not belief in the old gods, ie: freya, were 'real' beliefs that shaped people's lives or more myths and archetypal figures they lived their lives by. There are several references as late as 960 I believe that give serious weight to the intercession of gods, directly and manifestly in the lives of several people - bearing in mind that these are sagas, but understand that the icelandic saga is a history first, not a beowulf type structure. Very pragmatic. So it's an interesting contrast that the decision to 'go christian' is the only known vote of it's kind, I believe, and that it was dominated by pragmatic concerns and not due to a hard core belief in the spiritual power of christianity - it also wasn't being pushed by proselytizers from outside powers, such as in SE asia or Japan.