Right-believing Grand Prince Mstislav I Vladimirovich the Great, of Kiev

CORRECTION: the date "April 15", previously listed as New Style, is actually Old Style; the New Style equivalent has been corrected to April 28.

Right-believing Grand Prince Mstislav, baptized Theodore (also called Harald after his maternal grandfather, Harold of Wessex), was born around June 1, 1076, in Smolensk, the eldest son of Vladimir Monomakh. From age 12 and for about 29 years he ruled Novgorod, campaigning against the Chuds, Livs, and other neighbors, and won the Novgorodians' love. In 1125, after his father's death, he became Grand Prince of Kiev, holding the other princes in strict obedience — a last period of relative unity for Kievan Rus'; his death in 1132 is traditionally regarded as marking the beginning of the state's final disintegration into independent principalities.

By his order the Mstislav Gospel was created; under him the icon that later became the Vladimir Icon of the Mother of God was brought to Rus'. He died April 14/15, 1132, and was buried in the Church of St. Theodore monastery he had founded in Kiev. He is venerated as right-believing; his memory appears in Serbian and Bulgarian liturgical prologs of the 13th–14th centuries, as well as in the Synaxis of Novgorod Saints.

His feast is kept on April 15/28.