For centuries in Florence, March 25th, not January Ist, was the start of the new year. The city didn't adopt the practise of starting the year on January 1st until 1750.
There is a large plaque in the Loggia dei Lanzi, which records the change. It had been decreed in November 1749 by the Grand Duke of Tuscany, Francesco Stefano (r. 1737-65).
March 25th is the Feast of the Annunciation, the celebration of the announcement by the Archangel Gabriel to the Virgin Mary that she would conceive and become the mother of Jesus, the Son of God.
Florence abounds with images of the Annunciation, but the most famous one of all takes the form of a fresco and can be found in the church of Santissima Annunziata. (which is dedicated to the Madonna of the Annunciation).
According to legend, the fresco was painted in the 14th century by Frate Bartolomeo. It would, perhaps, be more accurate to say that the fresco was mostly painted by the good monk, for the face of the Madonna is said to have been completed by an angel while he was asleep. Divine intervention gave the image of the Annunciation miraculous powers and Santissima Annunziata soon became an important place of pilgrimage.