The Battle of Matthew's Day (Estonian: Madisepeva lahing) was fought near Viljandi (probably in Vanamisa) on 21 September 1217 during the Livonian Crusade. The adversaries were the Sword Brethren (a German Crusading order) with their recently converted Livonian and Latgalian allies versus an army of 6000 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.

The Livonian crusade refers to the various military Christianisation campaigns in medieval Livonia – in what is now Latvia and Estonia – during the Papal-sanctioned Northern Crusades in the 12-13th century. The Livonian crusade was conducted mostly by the Holy Roman Empire and the Kingdom of Denmark. It ended with the creation of Terra Mariana and the Danish duchy of Estonia. The lands on the eastern shores of the Baltic Sea were one of the last parts of Europe to be Christianised.

On 2 February 1207, in the territories conquered, an ecclesiastical state called Terra Mariana was established as a principality of the Holy Roman Empire, and proclaimed by Pope Innocent III in 1215 as a subject of the Holy See. After the success of the crusade, the Teutonic- and Danish- occupied territory was divided into six feudal principalities by William of Modena.