Baltic Unity day (Lithuanian: Baltų vienybės diena, Latvian: Baltu vienības diena) is a commemorative day celebrated on September 22 in Lithuania and Latvia. It was recognized as such in 2000 by both the Seimas of Lithuania and Saeima of Latvia commemorating the 1236 Battle of Saule, where the joint pagan Samogitian and Semigallian forces decisively defeated the Livonian Brothers of the Sword.

Source – Wikipedia