From the wired.com article:
45 B.C.: Roman dictator-for-life Julius Caesar, alarmed that the calendar is growing out of whack with the seasons, adds an extra day to the month of February every four years.
Caesar was reforming a calendar based on 364 days, with an occasional extra leap month. But the Roman religious officials in charge of minding the calendar had been asleep at the switch, chronologically speaking. Caesar consulted with Egypt’s top astronomers, who told him the year was 365¼ days long. While he was making the fix, Julius also decided to give his name to the month of July.
Although Caesar decreed the new calendar in 46 B.C., that year had 15 months to make up for the accumulated discrepancy. The first add-a-day leap year was 45 B.C.
The new Julian leap day wasn’t added at the end of February originally, but on the day preceding the 6th of the calends of March. The Romans didn’t count the days of the months from 1 on up, but used an idiosyncratic system of calends, nons and ides — and we all know what happened to ol’ J.C. on the ides of March, 44 B.C.
Read the rest of the article here.