This article, which explains the difference between the traditional dating of the September Ember Days, and that currently used in the EF, was originally published in 2010. The discrepancy between the traditional rubrics and the 1960 version does not occur every year, but it does this year, and so I am republishing it for reference, adjusting the dates for 2014. You may also find interesting this article from last year on the September Ember Days .
The first Sunday of each of these months is the day on which the Church begins to read a new set of scriptural books at Matins, with their accompanying antiphons and responsories; these readings are part of a system which goes back to the sixth century. In August, the books of Wisdom are read; in September, Job, Tobias, Judith and Esther; in October the books of the Macchabees; in November, Ezechiel, Daniel, and the twelve minor Prophets. (September is actually divided into two sets of readings, Job having a different set of responsories from the other three books.) The “first Sunday” of each of these months is traditionally that which occurs closest to the first calendar day of the month, even if that day occurs within the end of the previous month. This year, for example, the first Sunday “of September” was actually August 31st, the closest Sunday to the first day of September, and the third Sunday of September was September 14th.
The Ember Days of autumn are the Wednesday, Friday and Saturday of the third week of September, during which the book of Tobias is read; according to the traditional system of calculation, this year they will occur on the 17th, 19th and 20th. The system is also calculated so that the Ember days will always begin on the Wednesday after the Exaltation of the Cross.
In the 1960 revision, however, the first Sunday of August to November is always that which occurs first within the calendar month. According to this system, the first Sunday of September was the 7th of the month, the third will be the 21st, and the Ember Days will be the 24th, 26th and 27th.
This change also accounts for one of the peculiarities of the 1960 Breviary, the fact that November has four weeks, which are called the First, Third, Fourth and Fifth. According to the older calculation, November has five weeks when the fourth of the month is a Sunday; according to the newer calculation, November always has four weeks. In order to accommodate the new system, one of the weeks had to be removed; the second week of November was chosen to maintain the tradition that at least a bit of each of the Prophets would continue to be read in the Breviary.