Santa Maria Maggiore is one of the four Patriarchal Basilicas of Rome. Its construction was ordered by Pope Liberio who – they say – on August 5th 356 saw the Virgin Mary in his dreams. She indicated him where to construct the church through a snow fall that happened on the Esquilino Hill (this is the reason why the church is also called by the second name of Saint Mary “ad Nives”, of the Snows). It seems, however, that the actual Santa Maria Maggiore Basilica was built under Sisto III who erected it and dedicated it to Mary’s motherhood, just after the Council of Efeso in 431. Source: rome.info