I hate, I despise your feast days, and I will not smell in your solemn assemblies.
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 - i:e., I will take no delight in the sacrifices offered (Genesis 8:21, "The Lord smelled a sweet savour;" Leviticus 26:31).
In your solemn assemblies - literally, days of restraint. Isaiah 1:10-15, "Incense is an abomination unto me ... the calling of assemblies, I cannot away with; it is iniquity, even the solemn meeting," etc., etc., 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.