"once a week" 是一个常用的短语，表示 "每周一次"。它来源于英语中一个时间限定短语的固定搭配，即 "once"（一次）+ "a/per"（每）+ 时间单位（如 "week"，"day"，"month" 等）。
例句：
I go to the gym once a week.
(我每周去一次健身房。)
We have a meeting once a week to discuss progress.
(我们每周开一次会讨论进展。)
She only shops for groceries once a week.
(她每周只买一次杂货。)