With September almost upon us, one might take a moment to consider the Ember Days that follow the Feast of the Exaltation of the Holy Cross (September 14).
Ember Days date from the 5th century and are four sets of days (three each) set aside for fasting and penance. Each set of Ember Days consists of a Wednesday, Friday, and Saturday. There is one set more or less at the beginning of each season: they follow the feast of St. Lucy (12/13), the first Sunday in Lent, Pentecost, and the Feast of the Exaltation of the Cross. For most of Church history, the Ember Days were simply the Wednesday, Friday, and Saturday immediately following those respective days.
The revisions to the general Roman calendar in 1960, however, involved a change in how Sundays were counted. This caused the September Ember Days to fall on a week later in some years than they otherwise would. The New Liturgical Movement explains the insider-baseball at work. That post contains links to others with more information on Ember Days.
The penitential practices associated with the Ember Days are no longer obligatory on anyone: 1983 CIC c. 1251 controls one’s obligation to fast and abstain under Church law. For those who wish to observe the former discipline voluntarily, the rules were simple: each Ember Day was a day of fast; Ember Wednesday and Ember Saturday were days of partial abstinence (meat permitted only at the main meal); Ember Friday was (like all Fridays) a day of complete abstinence.