|Covid-19|April 9 – After a bizarre weather phenomena of yellowish clouds and dust chokes the air around Hangzhou, Song Dynasty, China, obscuring the sky and sun, a fire breaks out at night in the southeast of the city, which continues into the next day. Fighting the flames is difficult due to limited visibility. When the fires are extinguished, it is discovered that an entire district of some 10,000 houses in the southeast of the city were consumed by the flames.
Mongol troops cross the Yalu River into Korea, then under the Goryeo Kingdom.
Italy: Emperor Frederick II promulgates the Constitutions of Melfi (also known as Liber Augustalis), a collection of laws for Sicily, as well as the edict of Salerno, regulating the exercise of medicine and separating the professions of physician and apothecary.
Llywelyn the Great launches a campaign against the Norman lordships in Wales.
Spain: The Castillans reconquer the city of Quesada.
Ardengus becomes bishop of Florence.
March 17 – Emperor Shijō of Japan (d. 1242)
Guo Shoujing, Chinese astronomer and mathematician
John de Warenne, 6th Earl of Surrey (d. 1304)
Roger Mortimer, 1st Baron Wigmore (d. 1282)
April 6 – William Marshal, 2nd Earl of Pembroke
June 13 – Anthony of Padua, Portuguese saint (b. 1195)
November 6 – Emperor Tsuchimikado of Japan (b. 1196)
November 17 – Saint Elizabeth of Hungary, daughter of Andrew II of Hungary (b. 1207)
Abd al-Latif al-Baghdadi, Arab physician and traveller
Jalal ad-Din Mingburnu, last ruler of Khwarezmian Empire
Year 1231 (MCCXXXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar.