The “Old Covenant” Sabbath would end according to prophecy (Amos 8:5).
The “Sabbath” spoken of is the Sabbatical Year and not the weekly Shabbat (Leviticus 25:1-7).
There would be darkness during the messiah’s crucifixion (Amos 8:9-10; Matthew 27:45; Mark 15:33; Luke 23:44-45, 27:45).
This entire chapter is a vision given to Amos which is speaking about disasters that would occur. This verse uses darkness as a metaphor for a sudden disaster during prosperous times. This has been regarded as a prophecy regarding the death of King Josiah (II Chronicles 35:23-24).
The festivals here are a reference to the time when all Judah and Jerusalem mourned for Josiah (II Chronicles 35:24-25).
The sun is a reference to the kingdom of the house of David which is compared to the sun (Psalm 89:37).