I keep the BVM 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 BVM Feasts P. 1213-1235. In those cases, I place BVM divider at the divine office being prayed (P. 1213-1235).
If you are directed to read from the BVM section then the BVM parts of the divine office replace and supersede the same parts in Ordinary/Weekly Psalter pages 903 - 1141.