Can a wizard delay spell preparation?

Question

My wizard has just woken from a long rest and is thinking about what spells to prepare for a new day of adventuring. The PHB says on p.114:

You prepare the list of wizard spells that are available for you to cast. To do so. choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell). The spells must be of a level for which you have spell slots.

And:

Preparing a new list of wizard spells requires time spent studying your spellbook and memorizing the incantations and gestures you must make to cast the spell: at least 1 minute per spell level for each spell on your list.

As a 1st level wizard with 16 intelligence I have 4 1st level spells I can prepare.

So my question is, can I prepare 3 spells in the morning and then wait until I see a situation that calls for a particular spell and spend 1 minute studying my spellbook to learn that spell, provided that I still have a 1st level spell slot left?

Answer 1

No, spells must be prepared when you finish a log rest.

The section on Preparing and Casting Spells for the wizard states:

you can change your list of prepared spells when you finish a long rest.

If it said something like "anytime after you finish a long rest" then sure. But as it's worded you can only change your list of prepared spells when you finish a long rest. Failure to do so leaves you with the same set of spells you had prepared the day before, not open slots to fill later.

This is also backed up by Jeremy Crawford, lead rules designer in this twitter exchange:

Lebedev Dmitry
^@LebMit
@JeremyECrawford Can spellcasters prepare spells not all at once, but leave some "slots" for preparing spells later during the day?

Jeremy Crawford
^{‎@JeremyECrawford}
You prepare your list of spells only at the end of a long rest.

Answer 2

Yes. Asked the same question to Mearls via Twitter:

Michael Foster
^{‏@MichaelFoster}
@mikemearls Can a wizard prepare spells at multiple stages of the day, assuming they take 1 min per level and normal level restrictions?
_{2:37 PM - 23 Mar 2016}

Mike Mearls
^{‏@mikemearls}
@MichaelFoster38 yes
_{4:54 PM - 24 Mar 2016}

Equally, the PBP doesn't state that all spell choices must be prepared immediately after a long rest. By preparing a single spell and leaving others open, one has initiated the changing of the spell list, and is thus able to continue making choices until a full spell list has been built, even if continuing to build the spell list happens at different parts of the day, when required.