Your Divination abilities have grown to the point where you can predict which spell you will need. When memorizing spells, leave one blank, or write  "predicted" counting as one memorized spell.

At any time during the day, you can instantly replaced "PREDICTED" with any spell you know but did not memorize outright.  You could for example wait until you come across a door, then say "I predicted this and memorized Knock".

 This selection lasts until your next long rest.  You can not change the spell once you select it.  You can not take this feat multiple times.