Can one do a spell a day late?
The short answer is: yes. Because, as long as one is proficient enough in spellwork and writing/creating their own, they can typically modify almost anything to suit. However, having said that, it really depends on what the spell is, what the day was – were there specific energies that were being harnessed that are really only present at that specific time/date, etc…
For example, a friend recently lamented that they’d missed the Equinox and another had said it was no big deal they could still celebrate. Which is true, for the most part, except if one is looking specifically to do balancing work or some other spell that requires the equality of day/night that is typically only found on the Equinox. So could you still do it the next day… sure, but it’s not going have the same ummph as doing it on the day.
Similarly, the Blood Moon/Eclipse that occurred last night – having spellwork that was centered around using the energy created by the eclipse itself will not be as effective if it’s done tonight. Sure there will still be the full moon, but it’s missing that something extra. In this case, if one were going to change the date, it would be better to modify the spell so that the eclipse energy was no longer a key component.
Of course for some people, things like correspondences, moon phases, planetary alignment, days of the week or other considerations aren’t a huge part of their magical practices – and that’s perfectly fine. So in those cases, doing something one day versus another may not make that much of a difference. However, if one does feel that those things (or the lack thereof) do have an impact, changing days can affect the energy of the spell and/or cause it not to work the way it was intended.
In the end, it’s going to come down to one’s own personal practices – whether they are solitary/eclectic or part of an established tradition that may or may not have specific guidelines for spellwork. If one typically places a strong emphasis on timing, then changing something even slightly can throw things off. If one is more flexible and able to work with whatever is available, then changes are more easily compensated for. The key in both is to make sure that the spell being used is customized for the person using it – don’t take something from a book and use it “as-is” if the conditions don’t fit. The more energy we put into making it fit our needs (including timing, if that is important) the better it will be in the long run, in terms of working the spell successfully.