From Catholic Review:
Each year on Aug. 5 the church liturgical calendar commemorates the dedication of St. Mary Major (in Italian, Santa Maria Maggiore), one of the five great ancient basilicas in Rome, and the place Pope Francis chose to be entombed. Why would that event be a feast day for the universal church? What is our connection to the dedication of a building that took place in Italy over 1,500 years ago?
The history of this church is rooted in the role of Mary as the Mother of God. Its story begins with a fourth-century legend.