Jamieson, Fausset and Brown on Amos 5:21

21. I hate, I despise--The two verbs joined without a conjunction express God's strong abhorrence.

your feast days--yours; not Mine; I do not acknowledge them: unlike those in Judah, yours are of human, not divine institution.

I will not smell--that is, I will take no delight in the sacrifices offered (Ge 8:21; Le 26:31).

in your solemn assemblies--literally, "days of restraint." Is 1:10-15 is parallel. Isaiah is fuller; Amos, more condensed. Amos condemns Israel not only on the ground of their thinking to satisfy God by sacrifices without obedience (the charge brought by Isaiah against the Jews), but also because even their external ritual was a mere corruption, and unsanctioned by God.