白露 is the 15th of the 24 solar terms (节气), marking the time around Sept 8-22 when nighttime cooling produces visible dew on grass. The name literally means 'white dew' - old Chinese cosmology assigned the color white to autumn.
history
The poet 杜甫 wrote the famous line 露从今夜白 ('the dew turns white from this very night') in 《月夜忆舍弟》 - playing directly on the term 白露 to evoke autumn loneliness.
