The Rorate Mass takes its name 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 Caeli Mass is a traditional Advent devotion wherein the Mass of the Blessed Virgin Mary for Advent is offered just before dawn. 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 brighter, illumined by the sun as our Faith is illumined by Christ.
Information from https://fssp.com/rorate-caeli-masses/