The system described above lasted some 650 years until the dictatorship of Julius Caesar. Caesar made January 1st the beginning of the year and set the days of the months to their current length. December, the original tenth month, was now the twelfth month. A day was added at the end of February every four years to account for the four hour difference between the calendar and solar year. Caesar also changed Quintilis to July to honor himself. Not to be outdone, his successor, Caesar Augustus, named August after himself.
What is the origin of the names of the months? March was named for Mars, the God of War; April from Aphrodite, Goddess of Love. May comes from Maia the mother of Mercury; June from the God Juno. January comes from the God Janus and February from Februa, the Roman festival of Purification.
Oddly, September was renamed Germanicus during the year 37 A.D. April. May, and June were renamed Neroeus, Claudius, and Germanicus during the year 65 A.D.