日本語を勉強しましょう

月と鼈 = つきとすっぽん (tsuki to suppon)
月 = つき　= moon
鼈　＝　すっぽん　＝　mud turtle

彼らは月と鼈の違いだ。
They are as different as day and night.

I don't know why this kotowaza is translated as 'As different as night and day'. Why don't they use 月と日 instead? If you know, please explain it to me. :)