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Year 1156 (MCLVI) was a leap year starting on Sunday (link will display the full calendar) of the Julian calendar.
- Sfax revolts against the Norman occupiers, and massacres the Christians found in the city.
- January 20 – According to legend, freeholder Lalli slays English crusader Bishop Henry with an axe, on the ice of Lake Köyliönjärvi in Finland.
- December 25 – King Sverker the Elder is murdered on his way to church, and is soon succeeded as king of Sweden by his rival, Eric Jedvardsson.
- Prince Yuri Dolgorukiy founds and fortifies Moscow.
- The Privilegium Minus elevates Austria to the status of a duchy, ruled by the Babenburgs family.
- Mosan artists create the Stavelot Triptych, a masterpiece of goldsmithing, as a reliquary to house purported pieces of the True Cross.
- A rebellion breaks out against William I of Sicily, and the Byzantine Empire, encouraged by Pope Adrian IV, invades Apulia. William II crushes the rebellion, defeats the Byzantine armies at Brindisi, and humbles the Pope at Benevento. The city of Bari is laid to waste for the coming ten years.
- Raynald of Châtillon sacks Cyprus.