Mysql获取每周间隔的用户列表

I want to write query to fetch user from table who register before week interval. For ex. todays date is 2017-08-17, then I need user who register on 2017-08-10, 2017-08-03,2017-07-27 and so on. Like this if todays date is 2017-08-20 then user will be register on 2017-08-13, 2017-08-06.I want to write query to fetch user from table