The Ember Fasts

At the beginning of the four seasons of the Ecclesiastical Year, the Ember Days have been instituted by the Church to thank God for the blessings obtained during the past year and to implore further graces for the new season. Their importance in the Church was formerly very great. They are fixed on the Wednesday, Friday and Saturday: after the First Sunday of Lent for Spring, after Whitsunday for Summer, after the Feast of the Elevation of the Cross (14th September) for Autumn, and after the Third Sunday of Advent for Winter. They are intended also to consecrate to God the various seasons in nature, and to prepare by penance those who are about to be ordained. Ordinations generally take place on the Ember Days. The faithful ought to pray on these days for good Priests. The Ember Days were once fastdays of obligation.
(The Roman Missal, 1962)

Tomorrow is Ember Wednesday in Advent.

On the Wednesday of Ember week in Advent, the Mystery of the Annunciation is commemorated by many Churches. The Mass is sung early in the morning. That Mass is sometimes called the Golden Mass, Rorate Mass or Messias Mass. On that occasion the Church is lit up, as a token that the world was still in darkness when the Light of the world appeared. The Mass is called the Golden Mass possibly because in the Middle Age the whole of the Mass or at least the initial letters were written in gold, or on account of the golden magnificence of the solemnity, or more probably on account of the special, great, "golden" grace which, at that time, is obtained by the numerours prayers. It is called Rorate Mass after the first words of the Introit of the Mass: Rorate Coeli; and Messias Mass because the Church, like Our Lady, expresses on that day her longing for the arrival of the Messias.
(The Roman Missal, 1962)

Rorate coeli, desuper, et nubes pluant justum: aperiatur terra, et germinet Salvatorem.

Drop down dew, ye heavens, from above, and let the clouds rain down the Just; let the earth open and bud forth a Saviour.