once a week:
一星期一次
一周一次！
例句：She writes to her parents once a week！
Each student has to hand in a composition once a week！
once = one time
once a week = one time a week.
twice a week = 2 times a week.
一周一次。
英文表达时间、地点时，因文化差异，习惯于从小到大来表达，跟中文相反。