Battle of Kulikovo Day is observed in Russia on September 21 each year, commemorating the significant Battle of Kulikovo that took place on September 8, 1380. This pivotal battle occurred at Kulikovo Field near the Don River, where the Russian army, led by Prince Dmitri of Moscow, achieved victory over the Golden Horde commanded by Mamai. Although this victory did not end Mongol rule, it marked the decline of their influence, allowing Russian principalities to begin uniting under Moscow's leadership. The observance of Battle of Kulikovo Day on September 21 arises from the transition from the Julian to the Gregorian calendar, which shifted the date of the battle. The day serves as a memorial to this important event in Russian history.