The Midrash states that God said to Avraham: “I am one and you are one. I will grant your descendants one single day on which they can atone for their sins – Hoshana Rabbah.”
Mateh Moshe explains that God told Avraham that should Rosh Hashanah be insufficient to atone for their sins, then Yom Kippur will atone. And if Yom Kippur does not, then Hoshana Rabbah will.
Today is Hoshana Rabbah.