A Rorate Caeli Mass is a special Mass celebrated during Advent. The name comes from Isaiah 45:8: Rorate Caeli desuper et nubes pluant justum (“Drop down from above, heavens, and let the clouds rain justice”). The verse is used during vespers during Advent to express longing for the Messiah. It is also used as the introit for the Fourth Sunday of Advent and the feast of the Expectation of the Blessed Virgin Mary (Dec. 18).
The Rorate Mass is a votive Mass in honor of Mary. It is usually celebrated in the very early morning or late evening, accompanied only by candlelight. It was popular during the Middle Ages, also called the Angelic Mass or Golden Mass. In some countries, people would walk to the church carrying lamps or candles. Some say the melting wax of the many candles reflects the justice raining down. (Plus, who doesn’t love pretty candlelight in the winter dark?)