But the 20th is the 3rd sunday this year... So how can it be the 4th? This is confusing.
In the pre62 system the 20th is the fourth Sunday. In the Breviary the first Sunday of the months August to November was the Sunday
nearest the
Kalends of the month, i.e. the first day.
So, this year, the first Sunday of September was actually 30th August (Pentecost XIV); the second Sunday 6th September (Pentecost XV); the third Sunday 13th September (Pentecost XVI). Following the changes introduced in Rubricarum Instructum the system changed and the first Sunday of the months August to November was always the first Sunday to fall on, after, the
Kalends.
The other way to work out the September Ember Days is to remember they are the Wednesday, Friday and Saturday after the feast of the Exaltation of the Cross. That was on Monday of last week so, traditionally, the Ember Days followed. However, if using the 1962 rubrics the Ember Days will be the first
full week after the Exaltation as 'Pax Vobis' explained above.