Caesar took a great deal of interest in the calendar, perhaps because in addtion
to being a great general, he was also a priest, and concerned that the Roman
calendar needed revision because important seasonal ceremonies were no longer
coordinated with the antiquated calendar. Read all about it at:

http://www.greenheart.com/billh/julian.html

And for lots more about the calendar:

http://serendipity.magnet.ch/hermetic/cal_stud.htm