After reading this question, I remembered an issue I've had with the Scrying spell for some time. Specifically, the spell states:
If a target knows you're casting this spell, it can fail the saving throw voluntarily if it wants to be observed.
How detailed does the target have to know? Is it enough to say "I'll scry on you sometime in the next year", or does it have to be as detailed as "on the 23rd Oct 1491 at 12:31:09, I will scry on you"?
If it isn't the latter, the target would either have to notice something when it's scryed upon, or alternatively decide to fail all scrying saves against scrying made by you.
How specific does the target's knowledge of your casting have to be, then?