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According to the online Catholic Encyclopedia (1917 edition):

Ember days (corruption from Lat. Quatuor Tempora, four times) are the days at the beginning of the seasons ordered by the Church as days of fast and abstinence. They were definitely arranged and prescribed for the entire Church by Pope Gregory VII (1073-1085) for the Wednesday, Friday, and Saturday after 13 December (S. Lucia), after Ash Wednesday, after Whitsunday, and after 14 September (Exaltation of the Cross).

So the Ember Days were, in fact, last week (i.e., 16, 18, and 19 September 2015).  The Masses of the Ember Days appear in the Missal after the 17th Sunday after Pentecost, but because Pentecost is a movable feast, they do not necessarily follow that particular Sunday.

Interestingly, I have a hand Missal published by the SSPX which also indicates the Ember Days were last week.  I also have a copy of a hand missal published in 1965 that has the first changes including the all-English Canon.  It also indicates that the Ember Days were in the week following the 14th of September.  Presumably, this missal would have included any changes ordered by John 23, but there are no changes in the timing of the September Ember Days shown.

Is it possible that that SSPX chapel simply made an error or is this change published on the SSPX calendar?  Does the FSSP celebrate the Ember Days?  Anyone have a Calendar approved by the FSSP?  In each of my missals, which include 1945, 1955, 1962, and 1965 missals, the Ember Days are listed as occurring the Wednesday, Friday, and Saturday after the 14th of September.  I believe the Ember Days were completely abandoned in the Novus Ordo.

Edit to add:  I just found an online FSSP source that indicates that the Ember Days in September this year are in fact moved to this week.  This is interesting since none of the published missals I have provide any information on this change.  I don't know where it came from or why these changes are not indicated in the Conciliar or SSPX missal.

As others have stated, this is a modification that was made by John XXIII in the 1962 missal.

The other three Ember Days are relatively easy to calculate, given that they follow a specific Sunday (Third Sunday in Advent, First Sunday in Lent, and Pentecost).  The autumn Ember Days, however, are a bit more complicated, because they follow the "third liturgical Sunday of September," which, believe it or not, is not as simple to figure out as it might seem.

Under the "old" was of calculating things, the first liturgical Sunday in September didn't actually have to occur in September; rather, it was the Sunday that occurred during the first week of September.  So, for example, this year, the first liturgical Sunday of September was actually August 30.  This would make the third liturgical Sunday of the month September 13, and thus the Ember Days would be the 16th, 17th, and 19th.

John XXIII modified this so that liturgical Sundays and calendar Sundays became the same thing.  So, the Ember Days followed the actual third Sunday in September, which was September 20, making the Ember Days the 23rd, 24th, and 26th.

A lot of people believe that the autumnal Ember Days are connected with the Feast of Exaltation of the Holy Cross, but this is not officially correct.  It is an often-cited rule of thumb, but officially the timing of the Ember Days has always been connected with the third Sunday of September.  It just a question of how you calculate what the third Sunday actually is.