What Heidi Kim Linh said above is completely right. 夕 means the evening and 夜 is the night. They refer to the different time of a day.

夕 is a kanji radical that symbolizes "evening" 夜 means something closer to "night" as in, the time after the sun sets and the night is dark. I've only seen 夕 used as a kanji in a compound word like 夕方 (early evening --- like around sunset time, just as the sun starts going down) or 夕暮れ (late evening --- when the sun is below the horizon after it sets but it's not night yet)