Green flash

"Happy New Year... maybe."

- Prunella in "Arthur's New Year's Eve"

The green flash is a phenomenon that, according to Prunella, supposedly occurs every year at midnight on New Year's Day. In "Arthur's New Year's Eve", she asks Arthur if he's going to stay up to watch this, but he doesn't understand. She realizes that he's never stayed up until midnight on New Year's Eve before, and therefore doesn't know about the green flash. She explains that every year at midnight, people gather to watch the sky for a most amazing green flash. If it happens, it means that the new year has successfully begun. If it does not happen, then it has to stay the same year for another whole year.

Theories

 * As D.W. stays up until midnight and witnesses no green flash, it is possible that the story was entirely a creation of Prunella to try to impress Arthur. Alternatively, it is possible that there really is a green flash, and its absence explains, in part, the discrepancies in the Arthur timeline.

Trivia

 * The green flash is used a scene-changer to cut to Arthur having a discussion with Francine, who tells him that there is no green flash, and the only thing that happens on New Year's is that you have to throw away your old calendars, before the New Year's Police check on you. "At least, that's what I hear."
 * A is a real optical phenomena that sometimes occurs right after sunset or just before sunrise. Usually occurring for only a second or two, these flashes have a number of causes.