The difference is due to the liturgical changes of Pope John XXIII. The 16, 18, 19 are according to the pre-John XXIII reckoning, and the 23,25, 26 according to the post 1961 reckoning.
The difference comes from the readjustment of the Liturgical First Sunday of the month in the breviary. At Matins, Scripture lessons are given for each month. In order to determine the First Sunday of the month in September, the rule pre-1961 was:
- identify the day of the week on which the first of September occurs
- if it is Monday to Wednesday then the Sunday before is the first Sunday
- if Thursday to Saturday, then the following Sunday
After John XXIII, the first Liturgical Sunday is the first Sunday of the month.
The Ember days are placed in the Third Week (liturgically speaking) of September.
So for 2009, Sept. 1 is a Tuesday. Therefore:
pre-1961: the 1st Sunday of September is August 30. The 3rd Sunday is September 13th, and therefore the Ember days occur on the 16, 18, 19.
post 1961: the 1st Sunday of September is September 6, and the 3rd Sunday is September 20, so the Ember days are 23, 25, 26.
The association with the Holy Cross is because before the 1961 changes, the Holy Cross would always fall between a Wednesday in the Second Liturgical Week of September and a Tuesday in the Third Liturgical Week, so it was a popular and easy way of remembering them.