The Rorate Caeli Mass is a traditional Advent devotion wherein the Mass of the Blessed Virgin Mary for Advent is offered just before dawn. In many instances families and individuals travel an hour or more, rising and arriving very early for this stunningly beautiful Mass. The interplay of light and darkness speak to the meaning of Advent and the coming of the Light of the world.
The Mass takes its title, Rorate Caeli, from the first words of the Introit, which are from Isaiah 45:8:
“Rorate, caeli, desuper, et nubes pluant justum, aperiatur terra, et germinet Salvatorem.”
“Drop down dew, ye heavens, from above, and let the clouds rain the just: let the earth be opened and bud forth a Saviour.”
The Rorate Mass is lit only by candlelight. Because it is a votive Mass in Mary’s honor, white vestments are worn instead of Advent violet. In the dimly lit setting, priests and faithful prepare to honor the Light of the world, Who is soon to be born, and offer praise to God for the gift of Our Lady. As the Mass proceeds and sunrise approaches, the church becomes progressively brighter, illumined by the sun as our Faith is illumined by Christ.
From FSSP Website : link here