A period of thousand years is a Millennium. According to the Gregorian Calendar. To clearly understand a Millennium you need to know A decade means ten years and a century means a hundred years and a millennium means a thousand. We can even find out Which millennium are we in now?
In contemporary history or past calculations, we are in the third millennium of the anno Domini or the Common Era according to the Gregorian calendar.
There was also a public debate regarding the celebrations of the year 2000 and there was confusion as to whether the beginning or start of that year should be marked as the beginning of the new millennium. According to History, there had been a debate about the previous decades, centuries, and millennia. Issues raised in calculating millennia and the difference between the convention of using ordinal numbers or original numbers to count years and millennia as in now it is "the third millennium"