The wizard class feature Arcane Recovery can be used "once per day."
Arcane Recovery You have learned to regain some of your magical energy
by studying your spellbook. Once per day when you finish a short rest,
you can choose expended spell slots to recover.
This seems open to multiple interpretations to me. The most obvious is that the short rest with Arcane Recovery at the end of it can't be used until after a long rest, however, any mention of a long rest is conspicuously missing. The next most obvious interpretation is that 24 hours (or a sunrise and a sunset, makes little difference to me) must pass before using it again.
During most adventures, these two interpretations are effectively the same. This would come into play on the occasion that a long rest was interrupted or skipped, especially several days in a row.
Which interpretation is most supported by the rules or developer commentary?
Best Answer
It is unclear, however I lean towards it meaning that you can use it once between long rests.
"Once per day" occurs three times in the PHB, two of which have since been errata'd to mean you can re-use it after a long rest (and must have a long rest in order to re-use it):
This leaves Arcane Recovery as the only power in the PHB to refer to "once per day" without referring to a long rest. This may be intentional, but while "once per day" is used many times in previous editions (e.g. in D&D 3.5), there has been an effort to replace it by meaning to "once per long rest" in equivalent features. This works nicely for consistency, plus in most circumstances, it works similarly, as you can take only one long rest in a 24-hour period.
Additionally, on page 11 of the Monster Manual (under "Limited Usage"), it specifies that for monster abilities, if it says "1/day", the monster must finish a long rest to use it again.
So while this may not be the rule as written, I think it is one of those things that should have been corrected to mean "once per long rest", but hasn't (yet).