The Church of St. Demetrios in Thessalonica was built over the site of his holy martyrdom (martyrium). The Roman bath in which he was imprisoned. This bath is preserved to this day in a crypt under the holy sanctuary of the Church. Saint Demetrius of Thessaloniki (Greek: Άγιος Δημήτριος της Θεσσαλονίκης) was a Christian martyr, who lived in the early 4th century. During the Middle Ages, he came to be revered as one of the most important Orthodox military saints, often paired with Saint George. His feast day is 26 October for Christians following the Gregorian calendar and 8 November for Christians following the Julian calendar.