Civilizations have always relied on sophisticated astronomical calendars to determine when to plant and harvest crops. Barbarians, however, mostly used their crude calendars to determine auspicious times for starting wars, i.e. so they could get out of the house when their wives had PMS.
The classical Mongolian calendar consists of nine thirteen day months followed by one six-hundred day month. Festivals coincide with the appearance of airplanes in the sky over southeast Kyrghizstan. Nowadays, these are celebrated by stir-frying bats and serving them with chocolate chips and crushed drywall.
The Bunglorian calendar has five three-day months followed by a 350-day leap year every year. This corresponds to the Gregorian calendar, except that every day of the entire calendar is a Tuesday. This calendar predicted the birth of Christ in 957 CE and the destruction of the planet Earth in 1994.
The only other known barbarian calendar, last used in the Hyborian age, has twenty-nine three-hour months, with a twelve-second month at 6:15pm on Arbor Day. Hours are seventeen days long, except during five random festivals where time decelerates, then stops and reverses until a child is born with creepy reptilian snake eyes. These are known as Füg Flügür Cycles and the last one ended upon the birth of James Spader in 1960 CE. By the Hyborian calendar, this event occurred 1246 years before the birth of Christ.
The Coligny Calendar, an ancient Celtic device, predicted the rise of Insane Clown Posse.