Radak: With affliction, there is no [Rosh] Chodesh or Shabbos. Rosh Chodesh and Shabbos, which have Korban Musaf, were days of Simchah. The Mo'adim were days of rest and Simchah. They will not have Simchah, due to many afflictions.

Malbim: They used to celebrate these days to foreign service, for they thought that these are days of success; the stars will influence good and Brachah. I will cease these days ? they will be reversed to affliction and darkness. From this, she will realize that her good and happiness are not in the hands of the Ma'arachah, only in Hashem's hands; He switches the times and arranges the stars as He desires.

Radak: Also Yechezkel said "uv'Chagim uv'Mo'adim" (46:11). There is a Chag that is not Mo'ed, only Simchah ? a person rejoices, eats and drinks and it is called Chag, e.g. "va'Ya'as Shlomo ba'Es ha'Hi Es ha'Chag? Shiv'as Yamim v'Shiv'as Yamim" (Melachim I, 8:65) ? the verse calls the seven days of Chinuch [ha'Bayis] Chag. It says also "Ochelim v'Shosim v'Chogegim" (Shmuel I, 30:16). There is Mo'ed that is not Chag, e.g. "l'Osos ul'Mo'adim", "la'Mo'ed Ashuv Elecha" (Bereishis 1:14, 18:14), or any time that one sets for his friend, e.g. "ha'Mo'ed Asher Ye'ado" (Shmuel II, 20:5).

Radak citing Rav Sadya Gaon: "Chag" refers to the three festivals. "Mo'adim" refers to Rosh Hashanah, Yom Kipur, and Shemini Atzeres ¹ .