In the sense of the Common Era calendar the third millennium began on January 1, 2000 and will end on December 31, 2999. The majority of the celebrations for the start of the third millennium occurred at midnight on December 31, 1999 / January 1, 2000. The first decade of the new millennium (see below) contains the year 2000.
The third millennium is the third period of one thousand years in the Common Era.
September 11, 2001 attacks are one of the few events of the progressing 3rd Millenium that have had significant and polarizing effects on international affairs and relations.
Some millennialists and Christian anarchists predict the third millennium will be the millennium of peace, when mankind finally learns to live in harmony with each other and nature.
There was also a third millennium BC, which began (by the modern calendar) on January 1, 3000 BC and ended on December 31, 2001 BC.
While human lives may be measured in decades, and historical events in centuries, a millennium is so long a period by the scale of human activities as to be of little value as a measurement of time.