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?
Best Answer
No, spells must be prepared when you finish a log rest.
The section on Preparing and Casting Spells for the wizard states:
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: