In the post-conciliar calendar of Pope Saint Paul VI, today is the feast of the three Archangels, Saints Michael, Gabriel, and Raphael. But, while an aspect of this feast has always honored all the angels (cf. the Collect of the Mass), today's feast has long been associated primarily with one angel in particular, Saint Michael the Archangel. Hence today's traditional English name Michaelmas.
The feast originated in the 5th-century dedication of a Roman Basilica of Saint Michael the Archangel on the Via Saleria. Hence, its official title, prior to the post-conciliar calendar reform, was "The Dedication of Saint Michael the Archangel." Like many other great feasts it was at one time a holy day of obligation. (Its proximity to the autumn equinox also made it one of the "quarter days" in medieval England when financial accounts came due.)
The biblical readings from Daniel and Revelation used in the current Mass also highlight Michael, with no mention of the other two recently recently added archangels Gabriel and Raphael (who used to have their own separate, if lower ranked, celebrations on March 24 and October 24 respectively.) Michael's prominence in the Old Testament is suggested by the his image as Israel's defender - her "prince" (Daniel 10:21) and is reinforced in the New Testament as the one who leads the heavenly hosts in combat against Satan and defeats him (Revelation 12:7-9).
It was obviously this which inspired the famous 19th-century "Prayer to Saint Michael," which eventually became one of the so-called "Leonine Prayers" which came to be recited by the priest (in the vernacular) after Low Masses (Masses with no singing), originally for the recovery of the papal states. It was the bellicose imagery of that frequently heard prayer that motivated me as a nerdy nine-year old to choose Michael as my confirmation name!
But Michael has another aspect, which is the origin of his statue at the top of Hadrian's tomb in Rome, transforming it into the Castel Sant' Angelo. (Photo; the Castel Sant' Angelo with Saint Michael the Archangel at the top, taken by me in 2012.)
The legend is that, during a prolonged period of plague in 590, Pope Saint Gregory the Great led a procession on the occasion of which the pope had a vision of Saint Michael the Archangel atop the castle, wiping the blood from his sword on his mantle, and then sheathing it as a sign of the end of the plague.
As of yesterday, the global death toll from the coronavirus had reached 999,273, which means it will very soon surpass 1 million. Perhaps Michael and his feast could be repurposed as an occasion for intercessory prayer for health and safety in this post-modern, pandemic-prone world.