The fact that the solar year (of nearly 365 1/4 days) does not contain an exact number of days or of lunar months caused much trouble in keeping the calendar accurate. Before Caesar's time the method adopted was to reckon twelve lunar months (of 291/2 days to the year, and from time to time to insert an additional or thirteenth month to balance things. Through negligence, however, the official calendar was now two or three months ahead of the sun so that the spring equinox came in June. Caesar introduced the custom of regulating the year by the sun, not by the moon, and gave to each year 36 days, with an additional day inserted every fourth year. He also shifted the official beginning of hte year to January from march, where previous custom had placed it. In his honour the month of his birthday, hitherto called Quintillis, was renamed Julius (July). Similarly the next month , Sextilis, was later renamed after his successor, the emperor augustus.
(The story of Greece and Rome, J.C. Robertson and H.G. Robertson, Dent, London)