The Day That Never Comes is an extremely rare and spooky phenomenon that has only been reported by a handful of players in Grand Theft Auto V.
When this phenomenon occurs, the sun will set normally between 19:00 to 22:00 and night falls lasting for 24 hours. The moon will complete a cycle across the sky up to three times and the sun will fail to rise at the end of each.
Interestingly, all reports of this glitch claim it occurred on Saturdays. In addition, NPCs have been reported behaving bizarrely, such as walking in circles, avoiding each other, or casually starting fights with the player. Animals will freeze in place and birds will fly in a tornado pattern. It has also been noted that incomplete Random Events will not activate.
The Lunar Effect
This could just be a glitch, but it could also be a reference to the real world phenomenon, the Lunar Effect, which could have been added either surreptitiously or officially by a developer.
The Lunar Effect refers to the impact Lunar cycles can have on human and animal behavior, said to affect everything from birth-rates to menstruation and a particularly interesting belief, that during a full moon, certain people can display hysterical and psychotic behavior.
It is likely that this very rare occurrence is a glitch, as if it was a genuine scripted event, widespread reports would likely have surfaced. It is currently unknown what causes this glitch to occur and whether or not it has been fixed with subsequent patches.
It is most likely fake, with a lack of evidence, answers, and causes. There are few stories, no video evidence, and nothing seems to correlate with one another. It is almost unfalsifiable because of the elusiveness of the supposed occurrence.