Year 1380 (MCCCLXXX) was a leap year starting on Sunday (link will display the full calendar) of the Julian calendar. Events February – Olaf II of Denmark becomes Olaf IV of Norway, with his mother Margaret as regent. Iceland and the Faroe Islands, as a parts of Norway, pass under the Danish crown. May 31 – Grand Duke of Lithuania Jogaila signs the secret Treaty of Dovydiškės with the Teutonic Knights. This sparks a civil war with his uncle Kęstutis. July 27 –… (More on 1380)