The Missa de Angelis (Mass of the Angels) is Mass VIII in the traditional Kyriale. For a basic explanation of the Kyriale, see my post on the Mass Lux et Origo. It is believed to be newer than the most of the other Masses in the Kyriale, having been written at some point in the 14th or 15th centuries. While the Kyriale specifies this as a Mass for 2nd class feasts (in the EF) or just feasts (in the OF), this ordinary is a favorite of many congregations since it sounds like it was written in a major key, and is often sung on Sundays. This is one of the few ordinaries in the Kyriale that was not based off of a medieval trope.