This is the beautiful introitus of the Rorate Mass, which is a special Advent celebration in honour of the Virgin Mary.
Especially in German speaking countries it has been a long standing tradition to celebrate this votive Mass in the early hours of the morning during the weeks leading up to Christmas. Since Vaticanum II the Mass can also be celebrated in the evening.
Rorate caeli desuper, et nubes pluant iustum: aperiatur terra, et germinet Salvatorem. Ps. Caeli enarrant gloriam Dei: et opera manuum eius annuntiat fimamentum. V. Gloria Patri.