Some people say that it was on December 31st 2000.
They point out that the calendar runs from 1 BC A AD without the yera 0 is between. So, two thousand years since the birth of Christ were over only at the end of the year 2000.
They may be right, but most people celebrated the last day of the second millennium on Cecember 31st 1999. However, the millennium ended at different times in different parts of the world. The first place to witness the last day of the old millennium was the Caroline Islands belonging to the Republic of Kiribati. These Islands lie in the West Pacific, and are 14 hours ahead of Greenwich Meantime. The last place in which the Old Millennium lingered was Ebon Atoll, which is part of the Republic of Marshall islands. Ebon Atoll is 12 hours behind Greenwich Mean time.