The Shabbat between Rosh Hashanah and Yom Kippur is called Shabbat Shuvah—the Shabbat of Return. This name is derived from the Haftarah (reading from the Prophets) for this Shabbat, which begins with the words (Hosea 14:2) "Return, O Israel, to the Lord your God...".
Since Shabbat belongs to the 10 days of repentance, it is an auspicious time to rectify past mistakes and missed opportunities and positively influence the coming year.
The great Kabbalist Rabbi Yitzchak Luria (known as the Arizal) taught that the seven days between Rosh Hashanah and Yom Kippur (which always include a Monday, a Tuesday, etc.) correspond to the seven days of the week. The Sunday between Rosh Hashanah and Yom Kippur encompasses all the Sundays of the year, the Monday encompasses all the Mondays of the year, and so on. Shabbat Shuvah is, therefore, the potential of all the Shabbatot of the year.