In the broadest sense, this type of thinking was called millennialism - the idea that at the end of the world Christ would return for 1,000 years before final judgment. The calculation for this date varied, and some, including Pope Sylvester II, included the year 1000 as a possible date. These dates were calculated using various methods of medieval numerology. So the date was only significant insofar as it agreed with prophetic calculations.

Here are some sources that might help

1. The year 1000 : religious and social response to the turning of the first millennium by Michael Frassetto (Palgrave, 2002)

2. The feast of Saint Abraham : medieval millenarians and the Jews by Robert E Lerner (U Penn, 2001).

3. The pursuit of the millennium : revolutionary millenarians and mystical anarchists of the Middle Ages by Norman Rufus Colin Cohn (1961) -- old, but a classic on the subject.

4. Joachin of Fiore & the prophetic future : a medieval study in historical thinking by Marjorie Reeves (1999)

5. Dominion of God : Christendom and apocalypse in the Middle Ages by Brett Edward Whalen (Harvard, 2009)

6. Medieval numerology : a book of essays by Robert Leo Surles (Garland, 1993)

7. Astrology and numerology in medieval and early modern Catalonia : the Tractat de prenostication de la vida natural dels hòmens by John Scott Lucas (Brill, 2003)

Hope this helps a little. Happy reading!

Most of the reactions regarding the change from 999 to 1000CE comes centuries later after the event, with the earliest being seen in the writings of German abbot Joannes Tritemius. His writings contains bits about it being the end of days, fear of the population. However seeing how almost all of the writings about the reaction are after the date, it's impossible to truly say if they reacted the same. Seeing how later reactions to change in the centuries went, it's safe to bet that they probably panicked too.

A similar question was asked here, you can also try to grab a hold of Richard Erdoes' A.D. 1000: Living on the Brink of Apocalypse

hi! you may be interested in this post; it includes a link to the FAQ

• Were there any "millenium hysteria", doomsday prophecies or such at the shift 1000 AD?

Weren't they still using the Julian calendar at the time? I seem to recall there being a good deal of conflict among proponents of various calendar systems. So anyone seeking to celebrate the new year would have to argue that it really had been 1000 years since Christ.