The Sabbath of Returning and Repentance
The Shabbat that falls during the week between Rosh Hashanah and Yom Kippur is called Shabbat Shuvah, or the Sabbath of Return, but Shabbat Shuvah is also a pun.
Shuvah, sounds very much like teshuva, or repentance, another core concept of the High Holidays.
With Yom Kippur, and the Book of Life foremost on everyone's minds, the services this Shabbat and the atmosphere are solemn and focused. The haftarah portion is made up of selections from two books of Prophets--Hosea, and either Micah or Joel, depending on whether the community is Sephardic or Ashkenazic. Ashkenazim read Hosea 14:2-10 and Joel 2:15-27. Sephardim read Hosea 14:2-10 and Micah 7:18-20. The selection from Hosea focuses on a universal call for repentance, and an assurance that those who return to God will benefit from Divine healing and restoration. The selection from Joel imagines a blow of the shofar that will unite the people for fasting and supplication. Hosea focuses on Divine forgiveness, and how great it is in comparison to the forgiveness of man. Other than the special haftarah, the service on Shabbat Shuvah is not any different from a regular Shabbat.
Historically, Shabbat Shuvah was also a time when the rabbi of the community would present a sermon to the congregation. Though this is done every week in most contemporary communities, in previous eras a rabbi's sermon was expected only twice a year: on Shabbat Shuvah, and on Shabbat Hagadol, the intermediate Shabbat of Passover. Sermons on Shabbat Shuvah traditionally focus on themes of repentance, prayer, and charity.