Some call the eleventh of Tishre "b'shem Hashem" ("בשם ה׳"). (I assume this is a hasidic term, since, according to what I've heard, the name comes from the Baal Shem Tov.) Why is it called that?
The various names of Hashem are means by which created beings relate to Him; His own Essence is beyond any names. Our sins cause a blemish in the name Havayah, and during the time of forgiveness of sins [Aseres Yemei Teshuvah, or maybe more broadly also including Elul] we rise up to the level of "before [i.e., above] Havayah." Thus, the day after Yom Kippur can't be associated with any specific divine name - it's just "Hashem's Name."
In a footnote there, the Rebbe points out that Eshel Avraham (Buczacz, Orach Chaim 624:5) gives a different reason, also in the name of the Baal Shem Tov: because on this day we once again begin mentioning Hashem's name where we had omitted it before, saying הא-ל הקדוש instead of המלך הקדוש.
He concludes that "these two explanations may be reconciled," but doesn't suggest how.