The whole world will be celebrating the beginning of the third millennium on January 1, 2000, but if you had done a little mathematics, you may see things differently. Then you would realise that the actual start of the next millennium is not in 2000, but a year later.
But you will not be skipping parties just because you know that the new millennium starts in 2001. Since we base our maths on our 10 fingers, the year 2000 does represent a time when a year ends in all zeros.
But January 1, 2000 is not the start of the new millennium. The calendar that brings us the year 2000 was created by a monk called Dionysis Exiguus in the sixth century. He thought that there was a need to standardise the calendars so that people would know what day Easter falls on. Therefore, he decided to use Jesus' birthday as the first year (note that he did not start to count from zero). At that time, Jesus' year of birth was not exactly known, but the monk had a good guess, then counted up the years to his present time.
Dionysis started the first millennium at the beginning of 1 AD, so the second millennium began on 1001 AD, and the third must begin on 2001 AD.
If anyone thinks that something bad is going to happen 2000 years after Jesus' birth - which was about six years earlier than 1 AD - then whatever that was supposed to happen would have occurred about five years ago.