The Julian Calendar is the old calendar of Europe, almost completely replaced by the Gregorian Calendar. In some outlying parts of Shetland, it is still used to calculate some of the Merkis Days, such as New Year's Day and Christmas/Yule, notably Foula.
It is largely considered inaccurate, since it makes less use of leap years etc. Since the orbit of the Earth around the Sun is not exactly 365 days long, but slightly over by eleven minutes, use of the Julian Calendar created a "drift" of days, meaning that Julian time ended up not matching the actual solar year after a few centuries.
It is named after Julius Caesar, who had the calendar created in about 45 BC.
In addition to its use in Shetland, Orthodox churches still use the Julian calendar. Its use in secular Russia existed right up to the Revolution in 1918. The UK officially adopted the Gregorian Calendar in 1752.