"The Rorate Caeli Mass is a traditional Advent devotion wherein the Mass of the Blessed Virgin Mary for Advent is offered just before dawn. 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.
The readings and prayers of the Mass foretell the prophecy of the Virgin who would bear a Son called Emmanuel, and call on all to raise the gates of their hearts and their societies to let Christ the King enter; asking for the grace to receive eternal life by the merits of the Incarnation and saving Resurrection of Our Lord." Excerpted from the FSSP website, January 8, 2013