In his work, The Restoration and Organic Development of the Roman Rite, the late Professor Laszlo Dobszay had this to say about the Ember Days and about this effective loss:
The abolition of the Ember Days was the destruction of a very early tradition. We learn from the sermons of Leo the Great how devotedly the Roman Church kept this observance in the fifth century. 'Et traditio decrevit, et consuetudo formavit' - 'inasmuch as tradition has decreed, so custom shaped it' - said this most liturgical pope. And the same sermon proceeds so: 'ideo ipsa continentiae observantia quattuor est assignata temporibus, ut in idipsum totius anni redeunte decursu, cognosceremus nos indesinenter purifactionibus indigere...' - 'therefore four times are assigned for the observance of temperance, so that when the course of the year brings it back, we should understand, that we are in need of ceaseless purification'.
The roots of the Ember Days stretch back to the Old Testament.
We have covered the matter of Ember Days much over the years, so here is some suggested reading, or re-reading as the case may be, which speaks to the matter. I would encourage you to read these and other such pieces from other sources; the Ember Days are one of the great treasures of our Roman patrimony.
Ember Days: Explanation and Two Proposals (Sept. 23, 2008)
Quatuor Tempora: Advent Embertide this Wednesday, Friday and Saturday (Dec. 15, 2009)
Ember Wednesday in Advent (Feria Quarta Quatuor Temporum Adventus) (Dec. 16, 2009)
The Golden Mass of Ember Wednesday (Dec. 16, 2009)
The scriptural readings for the Advent Ember Days see the lessons being particularly drawn from the book of the prophet Isaiah; on Ember Saturday, also Daniel. The Gospel readings are respectively focused on the Annunciation (Luke 1:26-28), the Visitation (Luke 1:37-47), and the exhortation of St. John the Baptist to "prepare ye the way of the Lord" (Luke 3:1-6).