A millennium is a period of 1000 years. When endorsed by the Venerable Bede two centuries later, the method of reckoning the past as BC and AD was formally adopted.
Lets look at the first decade.
Note the first year is one not zero. Normal methods of counting have the concept of a zero value. When the calendar was organized by Dionysius Exiguus, the year zero was not defined. Thus the whole calendar is shifted by one. The second decade starts at the beginning of year 11.