The Battle of Matthew's Day () was fought near Viljandi (probably in Vanamõisa) on September 21, 1217 during the Livonian Crusade. The adversaries were the German crusading order the Sword Brethren with their recently converted Livonian and Latgalian allies and an army of 6000 Estonian men from different counties, led by Lembitu, who had attempted to unify the Estonians. The Germans won, Lembitu and Caupo of Turaida (converted Livonian chieftain, fought on the order's side) were killed. Also many others were forced to convert.