I keep the Festival divider tab in the back of the book until I need it.
A SEASON or SAINT'S day will often direct you to read parts from the Festival Psalter P. 1192-1212. In those cases, I place the Festival divider at the divine office being prayed (P. 1192-1212).
If you are directed to read from the Festival Psalter section then the Festival parts of the divine office replace and supersede the same parts in Ordinary/Weekly Psalter pages 903 - 1141.