Okay, lifted this right from an adventure I’m writing for a friend’s world, but hey, mythology can jump worlds, right? 
Old myths tell of the great gods that began all things. Once they were joyous and content with one another. But over time, they envied the others’ powers. Wars broke out and tore the land asunder. The moon, their mother, wept tears of amber and her face became a pale yellow. The gods stopped their fighting when they saw this and had great sorrow, they wept for those they had slain and celebrated the lives of those who remained.
Once every few years the moon can be seen a pale yellow. The astrologers predict this and there is a great celebration in remembrance of the old tale, though few believe it.
These days, many merchants use it as an excuse to push their wares, temples hold rituals, and many guilds and nobles hold great feasts.