Who invented the Julian calendar?
The Julian calendar is named after Julius Caesar, who had a new method of calculating the year created and adopted it in 46 BC. This wasn't exactly the same Julian calendar that we know today, as further changes would be made to it. Eventually the old ten-month Roman calendar became a 12 month one, with the new months of July and August named after Julius and Augustus Caesar. Then much later, in the 6th century AD, a monk called Dionysius Exiguus (this translates as 'Dennis the Short'!) made more changes such as the AD (Anno Domini, or Year of Our Lord) system. However, there was some confusion as to exactly how he did his calclulations.
Today the Julian Calendar is only used in some Orthodox Christian countries, most notably Russia where Christmas is celebrated on 6 January. In most of the Christian world, the Julian was replaced by the Gregorian in the Middle Ages. You can learn more about these calendars here.