This ceremony is held every year on the anniversary of the heroic resistance of the Swiss Guard in defense of Pope Clement VII (1523-1534) when the protestant troops of Charles V sacked Rome on May 6, 1527.
What happened on May 6, 1527?
I heard a homily by Father Jim Doran, OMV, about it at Mass today. Following up on a google search? I found precious little. I mean, I learned that most of the Guard were slaughtered by the troops of Holy Roman Emperor Charles V (please read the words "Holy" and "Roman" judiciously.)
They came into Rome and they killed people—not just the brave guards, but lay folk, nuns, priests. Thousands of people. Perhaps as many as 12,000 people. Why? For the crime of being Catholic.
(So what else is new?)
Please pray for His Holiness, Pope Francis, and the Swiss Guards.
Martyrs of The Sack of Rome, pray for us.