expression, i-adjective
you better watch your step!; sleep with one eye open!
A proverbial warning that good times don't last forever; literally 'it's not always a moonlit night'. Used to caution someone that they can't always expect things to go smoothly.
「月夜の晩ばかりじゃない」と言って、父は私に気を引き締めるように言った。
Saying 'It's not always a moonlit night,' my father told me to stay on my toes.