From Catholic News Agency:
Advent is a season filled with rich Catholic traditions, but a slightly lesser-known one is growing in popularity among younger Catholics.
The ancient liturgy of the Rorate Caeli Advent Mass honors the Blessed Virgin Mary through a Mass celebrated at dawn, in complete darkness, and lit only by candles, which symbolizes Christ, the Light of the World, entering into the world with Mary as the vessel. Emerging in the Middle Ages, the Rorate Caeli Mass gets its name from the prophecy of Isaiah. Rorate Caeli is Latin for “drop down, ye heavens.” These are the opening words of this liturgy’s Introit, which is used as an opening psalm or entrance antiphon and comes from Isaiah 45:8.