Original: It became the eighth month either when January and February were added to the beginning of the year by King Numa Pompilius about 700 BC or when those two months were moved from the end to the beginning of the year by the decemvirs about 450 BC (Roman writers disagree).
Simplified: It was the eighth month when January or February were added to the start of the year by King Numa Pompilius about 700 BC.