ある晩

one evening

→ 或る
phrase Rare #39,624
one evening Used to introduce an unspecified but particular evening, especially in storytelling. Depending on context, it can also mean “one night.”
JA: ある晩、突然友達が家にやって来た。
EN: One evening, a friend suddenly came to my house.

ある晩ひとりの商人帰り道にその坂を登っていた

One evening, a merchant was walking up that hill on his way home.

ある晩家にいたら-帰宅した姉が変だった

One night, while I was at home, my older sister came home acting strangely.

— Carandiru (2003)

痛い思いをしてお産をしたある晩子供は死んだ

She endured a painful childbirth, but one night the child died.

— フォーリング・ダウン (1993)

ある晩俺が店に行くとそこにバーテンダーカルロスがいた

One night, when I went to the bar, Carlos the bartender was there.

— レザボア・ドッグス (1992)