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Year 1071 (MLXXI) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar.
- February 22 – Battle of Cassel: Robert the Frisian (son of Baldwin V) defeats his sister-in-law Richilde (widow of Baldwin VI) and her nephew Arnulf III, in a succession struggle for the County of Flanders, and is appointed count by King Philip.
- March – Zaynab an-Nafzawiyyah marries Yusuf ibn Tashfin, leader of the Almoravids, and becomes his queen and co-regent.
- April 15 – The Siege of Bari ends when Bari, the capital and last Byzantine-controlled city in the Catepanate of Italy, is captured by Italo-Norman forces under Robert Guiscard.
- August 26 – Battle of Manzikert: The Byzantine Empire loses to an army of Seljuq Turks, led by Alp Arslan. Byzantine civil war results in the Turkish conquest of Anatolia.
- William I of England defeats Hereward the Wake's rebellion, on the Isle of Ely.
- Edwin, Earl of Mercia rebels against William I of England, but is betrayed and killed, leading to the re-distribution of land within Mercia to William's subjects.