At that hour of the Sacrifice, at the words of the Priests, the heavens are opened, and in that mystery of Jesus Christ, the choirs of angels are present, and things below are joined with things on high, earthly things to heavenly, and the service is both a visible and invisible event.
Pope St Gregory the Great, Dialogue 1. IV. c. 58
(in Migne ed. III. 425 sq.): "Quis fidelium habere dubium possit, in ipsa immolationis hora ad sacerdotis vocem coelis aperiri, in illo jesu Christi mysteria angelorum choros adesse, summis ima sociari, terrena coelestibus jungi, unumque ex visibilibus atque invisibilibus fieri?"
[borrowed from a document of Fr John Mark McMonagle]