The paper puts forward a hypothesis about the date of the battle between a Novgorodian army of Prince Yaroslav Vladimirovich and a South-Russian army of Sviatopolk the Accursed on the Dnepr River at Liubech. According to chronicles, the battle took place when Dnepr began to freeze over. This evidence allows to refer the time of the battle to the second half of November — early December. On the eve of the battle Sviatopolk who had already stood in front of Yaroslav for three months drank overnight with his retinue. Sсholars tend to believe that Sviatopolk`s Christian name was Peter: images of St. Peter were stamped on his silver coins. Based on this a hypothesis is made that on the eve of the battle Sviatopolk celebrated his saint`s day: the memorial date of st. Peter of Alexandria falls on November 25th. Consequently, the Liubech battle itself which resulted in Yaroslav`s victory took place in the depth of night, before dawn, November 26th. And this very day of the victory over Kievan regime of fratricide Sviatopolk, memorable for Yaroslav, was afterwards chosen by him for the establishment of all-Russian feast of Victory, the autumnal st. George`s day.