Originally this feast was only for St. Michael, but then St. Gabriel and St. Raphael were added to the celebration.
The Bible mentions 3 archangels, Michael, Gabriel and Raphael. Each angel had a specific mission in salvation history, but they continue their protective help in the world, even though we may not see their invisible spiritual work.
The Roman Rite of the Catholic Church has assigned September 29 to be the feast of Sts. Michael, Gabriel, and Raphael, archangels. For many centuries this day was only reserved for St. Michael, as the Catholic Encyclopedia explains.
The Gelasian Sacramentary (seventh century) gives the feast “S. Michaelis Archangeli”, and the Gregorian Sacramentary (eighth century), “Dedicatio Basilionis S. Angeli Michaelis”, 29 Sept. A manuscript also here adds “via Salaria.” This church of the Via Salaria was six miles to the north of the city [of Rome]; in the ninth century it was called Basilica Archangeli in Septimo. It disappeared a thousand years ago.
September 29 became the principal feast of St. Michael, honoring the original dedication of the church on the via Salaria.
The feast of St. Michael on September 29 became in the Middle Ages a holy day of obligation, but the designation was eventually removed.
After the Second Vatican Council, the Church wanted to further highlight all 3 archangels and grouped them all together on September 29. Previously St. Gabriel and St. Raphael had separate feasts during different parts of the year.
Now they are celebrated on a single day, connecting them to the neighboring feast day of the Guardian Angels on October 2.