The Rorate Mass got its proper name from the first word of the Introit (Entrance antiphon): "Rorate caeli désuper et nubes pluant justum" ("Drop down dew, ye heavens, from above, and let the clouds rain the just"). In the extraordinary form of the Roman Rite, this Mass is celebrated very early in the morning on all Saturdays. In some areas, it is celebrated on several or even all weekdays during Advent (the Votive Mass of Our Lady in Advent). This custom faded after liturgical changes that occurred after the Second Vatican Council.
The Rorate Mass is a Votive Mass in honor of the Virgin Mary for the season of Advent. It has a long tradition in the Catholic Church, especially in German-speaking areas. The Masses had to begin relatively early in the morning when it was still dark due to winter-time and were said by candlelight.
This is a photo from Saint Benedict's Parish in Chesapeake, Virginia.