There are seven days between Rosh Hashanah and Yom Kippur.  The Arizal says that on each day of the aseres yemei teshuvah, one can repair all the errors and misdeeds he made on that day of the week through his life.  On Sunday of aseres yemei teshuvah, for example, he can repent and atone for every mistake he ever made on a Sunday. ~ Divrei Yechezkel