There are Four Sets of Ember Days, corresponding to Four Fasting periods, or generally so. There are one set of Ember Days in Lent, another set in Summer, another set in the Fall, and another set in Advent/Winter.
St. Leo the Great of Rome talks about the four fasting periods. The Third set, falling in Fall, used to be associated with what was called the Michaelmas Fast; the Summer set fell in the week after Pentecost Week, or the Wednesday, Friday and Saturday after Trinity Sunday, and thus helped inaugurate the Apostles' Fast. However, Gregory VII began the process of moving the Ember Days in Summer into Pentecost week, which is a major conflict, since it meant that the Ember days lost their meaning, since not only is fasting prohibited in Pentecost week, but, even abstinence; thus, beginning a process of disrupting the purpose of the Ember Days and their point as inaugurating a Fasting season.