Saint Bernard of Clairvaux was a prominent 12th-century monk and theologian known for his role in the Cistercian reform movement. He founded the monastery at Clairvaux in 1115 and became a key figure in promoting the Cistercian way of life, which emphasized simplicity and a return to monastic ideals. His writings on spirituality and theology greatly influenced medieval Christianity. In addition to his monastic work, Bernard was an important advisor to several popes and played a significant role in the Second Crusade. He was known for his eloquent preaching and deep devotion, earning him the title of Doctor of the Church. His feast day is celebrated on August 20.