But when are the Ember Days?

WHICH WEEK ARE THE EMBER DAYS?
According to the pre-modern versions of the Roman Rite, and the Book of Common Prayer, nd the Divine Worship Missal, the September Ember Days come after the festival of the Holy Cross. What a nice easy rule. A child can apply it. So that is where you will find them in the admirable Saint Lawrence Press ORDO (and in Prayer Book based ORDOs).

In other words, next week is Ember Week.

So why, in ORDOs printed according to the 1962 Roman books (LMS; SSPX), does the September Ember Week, this year, come a week later?

Technically, the reason why the Ember Weeks come where they do is that, in the Breviary, their readings are tied into those of the week after the Third Sunday of September. Before 1962, the "First" Sunday of September might actually be at the end of August. So, this year, August 30 was the official First Sunday of September. But the 1962 revisers changed this so as to be clear-cut and logical ... First Sunday of September for them has to mean literally First Sunday of September. Hence (if you're still interested) the Third Week of September starts September 13 according to the old reckoning, but not until September 20 according to 1962.

As so often happens when people try to tidy things up and to be neat and logical and clever, this decision of 1962 led to the potential dislocation of the Ember Week from its ancient mooring to Holy Cross Day.

IMPLICATIONS OF THIS
Since the 1962 rite lasted in widespread use less than a decade, I find it hard to take it seriously in those matters where it conflicts with what the Latin Church had kept easy and natural for centuries ... particularly when the Divine Worship Missal, the Ordinariate Altar Book, remains in line with the Missal of S Pius V.
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Summorum pontificum, I presume, took the 1962 books as normative for ecumenical and practical reasons: because this is what the SSPX had done since Archbishop Lefebvre changed his liturgical policy around 1974.

1962 should be regarded as an interim stop-gap.