From a book published in 1880, now available on Google books:
St. Gertrude saw our Lord Jesus Christ celebrate Mass in a mystical manner: On Gaudete Sunday [Third Sunday of Advent (Gaudete means "Rejoice")], as Gertrude prepared to communicate at the first Mass-----which commences "Rorate"-----she complained to Our Lord that she could not hear Mass; but Our Lord, Who compassionates the afflicted, consoled her, saying: "Do you wish, My beloved, that I should say Mass for you?" Then, being suddenly rapt in spirit, she replied: "I do desire it, O Beloved of my soul, and I most ardently beseech Thee to grant me this favor." Our Lord then intoned the Gaudete in Domino semper ["Rejoice in the Lord always"] with a choir of Saints to incite this soul to praise and rejoice in Him; and as He sat on His royal throne, St. Gertrude cast herself at His feet and embraced them. Then He chanted the Kyrie eleison ["Lord, have mercy"] in a clear and loud voice, while two of the princes of the choir of Thrones took her soul and brought it before God the Father, where she remained prostrate.