of Decemvir
Source: Webster's dictionaryFebruary remained the last month of the calendar year until the time of the decemvirs (c. 450 BC), when it became the second month. Source: Internet
It became the fourth month of the calendar year (the year when twelve months are displayed in order) during the time of the decemvirs about 450 BC, when it also was given 29 days. Source: Internet