I keep the Apostles divider tab in the back of the book until I need it.
A SEASON or SAINT'S day will sometimes direct you to read parts from the Apostles Feasts P. 1236-1249. In those cases, I place Apostles divider at the divine office being prayed (P. 1236-1249).
If you are directed to read from the Apostles section then the Apostles parts of the divine office replace and supersede the same parts in Ordinary/Weekly Psalter pages 903 - 1141.