Chol Hamoed refers to the “intermediate period” of the festivals of Passover and Sukkot. They are the days sandwiched between the beginning and ending holy days of both festivals.
G‑d told Abraham: “If atonement was not granted to your children on Rosh Hashanah, I will grant it on Yom Kippur; if atonement was not attained on Yom Kippur, it will be given on Hoshana Rabbah.”