People went nuts when 1999 was going to turn into 2000. The U.S. had feared that computers were going to crash because of the output of the year change, AKA the date, as technicians did not think about the turn of the millennium and left the "19" part of the year, so the last two digits remained.
So, did people react the same way in 999? The Gregorian calendar (the calendar most of the world uses) wasn't created until the 1580s, so did they even know about the change?
Much more can always be said if anyone has anything new to add, but for now we have some older answers about this:
Was the millennium from 999-1000 ad marked in any special way? by /u/savvysioux
Did people in 999 AD celebrate the new millennium? Were there any doomsday predictions similar to Y2K? by /u/haimoofauxerre