On the oracle bones, 夕 and 月 were drawn as the very same crescent — context alone told a reader whether the scribe meant "evening" or "moon". Only later did scribes start adding a short stroke inside 月 to set "moon" apart, leaving 夕 as the plain, bare crescent you see today: the sliver of moon that appears as daylight fades.
A piece of culture
The most important evening of the year 除夕 (chúxī), "New Year's Eve", is built from 夕 and is the single most important night on the Chinese calendar: families travel enormous distances to share a reunion dinner, elders hand out red envelopes, and firecrackers are set off at midnight — a tradition said to date back to scaring off the man-eating monster Nián, who fled from loud noise, fire and the color red.