Note: We also have "every other day" (每隔一天).
I have to see my dentist every three days. (我每隔三天就要去看一次牙医。)
However, "every single day" means every day. "Single" simply strengthen the repetitive sense.
Mom, why should I have to brush my teeth every single day? (妈妈，为什么我每天都得刷牙啊？)