Simplify the following text for a Grade 10 native English-speaking person.
Original: 1000 ( M ) of the Gregorian Calendar was the last year of the 10th century as well as the last year of the first millennium of the Christian era ending on December 31 .
Simplified: It was also the last year of the 10th century as well as the last year of the 1st millennium of the Christian era ending on December 31st , but the first year of the 1000s decade .