Ugh, I had a post and lost it!
I will attempt to explain. I am new to traditional Catholicism and I am SSPX. We use the 1962 Missal and the September Ember days this year are Sep 22, 24, and 25. I wasn't aware they had "shifted" on the calendar until I did my own research.
Ember Days fall at 4 times in the Liturgical Calendar. They are meant to coincide with the change of seasons and Catholics are obligated to fast, offers prayers, thanksgiving, and alms for the poor. These days, of course, were adopted to help ease the Pagans into the Church and the Church used the Pagans affinity for celebrating the earth and harvest to help them focus on Our Lord and God's goodness and power over the universe.
In Feb, this year, they fall between the first and second Sundays of Lent.
In May, they fall after Pentecost Sunday, which this year was 26, 28, and 29.
In Sep, they fall on the Wed, Fri, and Sat AFTER the 3rd Sunday of the month.
In Dec, they fall between the 3rd and 4th week of Advent.
Now here's the confusion. Because the Exaltation of the Cross falls on Sept 14, most people assumed that the September Ember days are related to that Feast Day. They are not.
Up until the 1960s, for liturgical purposes the Church calculated the first Sunday of the month as the Sunday that was closest to the first day of the month. Let me explain. This year, Sept 1 was on a Wednesday. The Sunday closest to Sept 1 was actually August 29th. So, prior to 1960, August 29th would have been the 1st Sunday of Sept. Pope John XXIII found this calculation system confusing, so he changed it to what is obvious. The first Sunday of the month is the first Sunday of the month. In other words, this year the first Sunday of September was Sept 5. Since September Ember days fall on the Wed, Fri, and Sat AFTER the 3rd Sunday of the month, those days are the 22, 25, and 25.
I mention this here because I think many who follow the liturgical calendar of the 1950s claim that Pope John XXIII was just playing around with the liturgical calendar. He may have been, but in this case he had a legitimate argument---he wanted to simplify how Sundays are calculated. It just so happened that it affected Ember Days.
Hope that helps.