D&D 5E Spell Preparation on Long Rest

[jasper]

I define "when you finish" to mean "at the moment a long rest is complete. Not a second before nor a second after".

As THE GRAND EVIL WIZARD GUACHI AWOKE AT 0602 HRS THAT MORNING, HE SNEEZED! So he was total unprepared for the adventurers knocking on his door at 0733 hrs because the day before his spells were set up for fun fun torture and tickle day, not the murder hobos going to murder me today day spells.

 

[Charlaquin1]

It would have made a difference to me as a wizard if I had to guess what spells I needed for the day as soon as I awoke or if I could wait until I had more information about what situations were immediately in front of me and select a more specialized spell.

I see, I misunderstood the question. I thought by “after” you meant immediately after. Seems to me like it goes without saying that the preparation has to be done immediately after a long rest, since if the character is more than a day old it’s literally always “after” a long rest. There wouldn’t be much point in calling out a long rest as important to spell preparation at that point, just say that you can only prepare spells once per day.

 

[Elon Tusk]

I see, I misunderstood the question. I thought by “after” you meant immediately after. Seems to me like it goes without saying that the preparation has to be done immediately after a long rest, since if the character is more than a day old it’s literally always “after” a long rest. There wouldn’t be much point in calling out a long rest as important to spell preparation at that point, just say that you can only prepare spells once per day.

Though I think the point is moot given the Sage Advice answer, the long rest requirement is different than a day requirement. Once per day requirements seem to be used for NPCs. A PC could go more a day without long resting.

 

[Tony Vargas]

The player's handbook states: You can change your list of prepared spells whenyou finish a long rest. Preparing a new list of wizardspells requires time spent studying your spellbook andmemorizing the incantations and gestures you mustmake to cast the spell: at least 1 minute per spell levelfor each spell on your list.
... that could mean "after."

Yes, it could. So, when you finish long rest of 8 hrs, you then append 1 min/spell level of prep time to that 8hrs. It's trivial when you're prepping first level spells, and there's not much point to prepping a lot of different high levels spells. Even at 5th, if you prepped mostly 3rd level spells, you'd be at it less than an additional half-hour. So, at low levels, it's likely to be ignored, and, by the time you reach levels where it might make a big difference will have likely fallen by the wayside.

So couldn't that mean that a wizard could take a long rest, head out toward a potential conflict, do some recon, and then stop just outside the conflict area to take a few minutes preparing spells that would likely be most useful?

No, because that's not "when he finishes a long rest," it's 'after he's gone to do some recon.'

Could she prepare a few spells just after the long rest and then some at a later point during the day?

I'd say that, yes, you could prep fewer spells than you're entitled too, leaving the remaining spells the same as they were the day before - but the rules don't actually say that, they seem to imply that you prep whole slates or nothing.
No, even were I ruling the above, you could not then change any of those remaining spells, nor 'blank out a spot' to prep a spell into later (that was an option in 3e, though, FWIW), because then you'd wouldn't be prepping 'when you finish a short rest.'

Does it matter if she has cast one or more spells prior to preparing them all?

Under the when=after interpretation, you have to have completed a long rest to prep your spells, it's the only time you get to do it.

The 'while' interpretation is simpler, and means the party doesn't have to wait around for you. Under the 'while' interpretation, if you decided to prep a lot a high level spells for some unknowable reason, it might cut into the portion of your long rest you'd use for sleeping....
...more restrictively, if you counted it against the hour of 'activity' before a long rest is interrupted a 60-spell-level limit might cramp the style of higher level casters, some of the time.

(very hypothetically, a 20th level 20 INT wizard who prepped 25 9th level spells - having presumably researched a lot of new 9th level spells for that sole purpose - would be at it for 3hrs 45 min)