The Ember Days come four times each years, just like the seasons that they mark. An accidental casualty of the revision of the Catholic Church's liturgical calendar in 1969, the Ember Days were made "optional," which meant that most Catholics would no longer practice them. That's a shame, because these quarterly calls to prayer, fasting, and abstinence served for almost 2,000 years to help Christians prepare themselves for the change of the liturgical seasons, as well as the natural ones.
Each set of Ember Days has its own character. In December, the Wednesday, Friday, and Saturday after the Feast of Saint Lucy (in 2013, December 18, 20, and 21) prepare "the people who have walked in great darkness" for the light that will come into the world at Christmas. Falling no earlier than December 14, 16, and 17, and as late at December 20, 22, and 23, they represent one last voice crying out in the wilderness, to make straight the way of the Lord in our hearts before we celebrate His first coming and look toward His second. The readings for the December Ember Wednesday—Isaiah 2:2-5; Isaiah 7:10-15; Luke 1:26-38—prophesy the preaching of the Gospel to the Gentiles and call us to walk in the light of the Lord, and recount Isaiah's prophecy of the virgin who shall give birth to God among us, and then show us the fulfillment of that prophecy in the Annunciation.
As the darkest days of winter fall upon us, the Church tells us, as the angel Gabriel told Mary, "Be not afraid!" Our salvation is at hand, and we embrace the prayer and fasting and abstinence of the December Ember Days—in the midst of the month-long secular party called "the holiday season"—not out of fear but out of a burning love of Christ, which makes us want to prepare ourselves properly for the feast of His birth.