A crescent moon, drawn almost identically to 月 but with one stroke fewer — the moon showing faintly while it is still dusk rather than full night. From the moon at that hour came the word for the hour itself.
除夕 is New Year's Eve, the night the old year is 'removed'. 七夕 is the seventh evening of the seventh month, the one night the Cowherd and the Weaver Girl cross the Milky Way to meet — China's own lovers' festival. 夕阳 is the setting sun, and 夕阳红 a gentle phrase for old age.
A thin crescent MOON showing at DUSK — 月 with one stroke taken away.
夕 is 月 minus one stroke, and 多 (many) is simply two 夕 stacked up — evening after evening after evening.