Livonian Crusade: The Estonian leader Lembitu and Livonian leader Caupo of Turaida are killed in the Battle of St. Matthew's Day.

The Livonian crusade consists of the various military Christianisation campaigns in medieval Livonia – modern Latvia and Estonia – during the Papal-sanctioned Northern Crusades in the 12th–13th century.

Lembitu was an ancient Estonian senior (elder) from Sakala County and military leader in the struggle against conquest of the Estonian lands by the German Livonian Brothers of the Sword at the beginning of the 13th century. He is the only Estonian pre-Crusade ruler, about whom some biographical information is known.

Caupo of Turaida, Kaupo or Kaupo Lieven, sometimes Kubbe was a leader of the Finnic-speaking Livonian people in the beginning of the 13th century, in what are now parts of Latvia and Estonia by the Gulf of Riga. He is sometimes called a 'King of Livonia', the Chronicle of Henry of Livonia refers to him as quasi rex, 'like a king.

The Battle of Matthew's Day was fought near Viljandi on 21 September 1217 during the Livonian Crusade. The adversaries were the Sword Brethren with their recently converted Livonian and Latgalian allies versus an army of 6,000 Estonian men from different counties, led by Lembitu, who had attempted to unify the Estonians. The Germans won, although the converted Livonian chieftain Caupo of Turaida died. Lembitu was also killed, and many other Estonians were forced to convert.

September 21 is the 264th day of the year in the Gregorian calendar; 101 days remain until the end of the year.