frequency expressions

She writes me a letter once a month.

She sends a letter to her pen friend in London twice a month.

Mail is delivered once a day.

She takes a cooking class once a week.

She plays tennis with her friends once a week.

She writes to her parents once a week.

She goes to that bookstore once a week.

She goes to the hospital once a week.

She went to that market once a week.

She does aerobics once a week.