Until the 1960's these were days of obligatory fast and full or partial abstinence. While no longer obligatory, this long-standing discipline would still be of merit to practice; indeed, I would take this opportunity to encourage our readers to take up this discipline tomorrow, Friday and Saturday.
Within the usus antiquior there are some interesting aspects to the Masses for these days, but for the moment, let us simply take a brief survey of the history of Ember days generally, by way of Josef Jungmann, the Catholic Encyclopedia, Bl. Ildefonso Schuster, Dom Gueranger and Fr. Francis Weiser.
Josef Jungmann, The Early Liturgy to the time of Gregory the Great:
These [the Ember weeks] are among the most ancient institutions of the Roman liturgy. We say quattuor tempora [the four times], but the most ancient sources of the Roman liturgy speak only of three such times. Three times a year a sort of retreat period was held... during the months of June, September and December one week was especially devoted to prayer and fasting. Wednesday and Friday were kept as days of fast, with the fasting continuing on Saturday. And then on Saturday evening a vigil was held in much the same way as the Easter vigil, with twelve lessons and with corresponding songs and prayers. This vigil service continued far into the night... These Ember weeks...were spaced three months apart, in the summer, in autumn and in winter...
The Catholic Encyclopedia:
At first the Church in Rome had fasts in June, September, and December; the exact days were not fixed but were announced by the priests. The "Liber Pontificalis" ascribes to Pope Callistus (217-222) a law ordering the fast, but probably it is older. Leo the Great (440-461) considers it an Apostolic institution. When the fourth season was added cannot be ascertained, but Gelasius (492-496) speaks of all four. This pope also permitted the conferring of priesthood and deaconship on the Saturdays of ember week -- these were formerly given only at Easter. Before Gelasius the ember days were known only in Rome, but after his time their observance spread... The present Roman Missal, in the formulary for the Ember days, retains in part the old practice of lessons from Scripture in addition to the ordinary two: for the Wednesdays three, for the Saturdays six, and seven for the Saturday in December. Some of these lessons contain promises of a bountiful harvest for those that serve God.
Blessed Ildefonso Schuster, The Sacramentary: Historical and Liturgical Notes on the Roman Missal:
The solemn fast of the three days in Ember Week seems to have been originally peculiar to the Roman Church, whence it was afterwards borrowed by the other Latin dioceses. St. Leo I explains its meaning clearly, especially on the occasion of the December fasts, when he remarks that, at the end of the year, and before beginning to draw upon the winter resources, it is very fitting that we should dedicate the firstfruits to the divine Providence by a freewill offering of abstinence and almsgiving. In this case there was a further motive. An ancient tradition reserved the ordinations of priests and deacons to the month of December, and the faithful -- following a custom introduced by the Apostles themselves -- felt constrained to unite with the bishop in prayer and fasting, in order to call down from God an abundance of priestly gifts upon the heads of those newly chosen to minister at the altar.
Dom Prosper Gueranger, The Liturgical Year:
This observance is not peculiar to the Advent liturgy; it is one which has been fixed for each of the four seasons of the Ecclesiastical Year. We may consider it as one of those practices which the Church took from the Synagogue; for the prophet Zacharias speaks of the fourth, fifth, seventh and tenth months... From the first ages, the Quatuor Tempora were kept, in the Roman Church, at the same time of the year as at present. As to the expression, which is not unfrequently used in the early writers, of The Three Times and not The Four, we must remember that, in the spring, these Days always come in the first week of Lent, a period already consecrated to the most rigorous fasting and abstinence... The intentions, which the Church has in the fast of the Ember Days, are the same as those of the Synagogue; namely, to consecrate to God by penance the four seasons of the year. The Ember Days of Advent are known, in ecclesiastical antiquity, by the name of the Fast of the tenth Month; and St. Leo, in one of his sermons on this Fast...tells us that a special fast was fixed for this time of the year, because the fruits of the earth had then all been gathered in, and that it behooved Christians to testify their gratitude to God by a sacrifice of abstinence... The fast of the Ember Days has another object besides that of consecrating the four seasons of the year to God by an act of penance; it has also in view the Ordination of Ministers of the Church, which takes place on the Saturday... The faithful should unite with the Church in this her intention, and to offer to God their fasting and abstinence for the purpose of obtaining worthy Ministers of the Word and the Sacraments, and true pastors of the people.
Fr. Francis X. Weiser, Christian Feasts and Customs:
In the earliest liturgical books the Ember Days are simply called "the fast of the first, fourth, seventh and tenth month" (that is, March, June, September, December)... During the sixth century the term Quatuor Tempora (Four Times or Seasons) was introduced, and has remained ever since as the official ecclesiastical name for the Embertides. From the Latin word most European nations coined their popular terms... The English term Ember seems to derive from the Anglo-Saxon ymbren (season, period).
Those of you who are interested in this subject may also find this article of interest, The Glow of Ember Days by Michael P. Foley, an associate professor of patristics at Baylor university.
His article was originally published in The Latin Mass Magazine in Fall 2008.