An excellent account of a Rorate Mass can be found at RORATE CAELI
Altar, with Violet Cloth and Antependium, for the Season of Advent,
prepared for the Celebration of a "Rorate Mass",
in which the Rorate Caeli is sung.
Photo: 3 September 2011.
Source: Own work.
Author: Kaiser Guilherme II.
The Rorate Mass got its proper name from the first word of the Introit (Entrance antiphon): "Rorate caeli désuper et nubes pluant justum". "Shower, O heavens, from above, and let the skies rain down righteousness". We know Gaudete Sunday and Laetare Sunday, which also got their names from the Introit. Before the Liturgical changes post-Vatican II, this Mass was celebrated very early in the morning on all Saturdays. In some areas, it was celebrated on several, or even all, weekdays during Advent (The Votive Mass of Our Lady in Advent).
The Rorate Mass is a Votive Mass in honour of The Blessed Mother for the Season of Advent. It has a long tradition in The Catholic Church, especially in German-speaking countries. The Masses have to begin in the morning, when it is still dark, due to winter-time, and are said by candlelight.