The Perpetual Calendar
Article about the process of switching from the Julian to the Gregorian calendar, with a table showing when each nation made the transition.
What is the Julian Calendar?
Explains the workings of the Julian Calendar, used by the Roman Empire after 46 B.C. and by the Catholic Church until the 16th century.
Wikipedia: Julian Calendar
Hyperlinked article about the calendar that was introduced by Julius Caesar in 46 B.C.E.