The First Woe, Concerning The Day of YHWH (Amos 5:18-27).
A ‘woe' was the opposite of a ‘blessed be you of YHWH'. It was the announcing of troubles and disaster that would come on His people directly from YHWH (compare Isaiah 5:8-30; Isaiah 10:1-4; and see Numbers 21:29; 1 Samuel 4:7-8) in fulfilment of His warnings (Leviticus 26:14-46; Deuteronomy 28-29).
It is apparent from the context that Israel were living in expectation of ‘the day of YHWH' (compare Isaiah 13:6; Isaiah 13:9), and that they saw it as something to look forward to, a day which would bring ‘light' (2 Samuel 23:4; Psalms 37:6; Psalms 112:4; Psalms 118:22-27) and worldwide domination (see Psalms 2; Psalms 22:22-31; Psalms 89:19-37; Isaiah 9:4-7). It was not in fact unusual for great kings to see themselves as destined by their national gods to rule the world. The kings of Assyria, for example, gave the credit for their victories to Assur whom they saw as having told them to go forth and subdue their world. It was as the representative of Assur that Sennacherib would proclaim his victory over all gods (except, as it turned out, YHWH). See 2 Kings 18:33-35. But in most cases this idea followed as a result of initial military success and of having a powerful army. It was different with Israel in that they considered this to be true long before they enjoyed military might, not because of their military might, but because of the superiority of their position as YHWH's chosen people, something evidenced by the call of Abraham, who had been told that all the nations of the world would be blessed (or would bless themselves) through his seed (indicating to the ancient world that he would rule over them), and that kings would come out of him (Genesis 12:3; Genesis 17:6; Genesis 18:18); by the deliverance from Egypt which had confirmed their special position (Amos 2:10; Exodus 19:5-6; Deuteronomy 32:8-9); and by the granting to them of the land of the Amorites (Amos 2:10; Exodus 23:23; Numbers 21:31; Deuteronomy 1:20; Deuteronomy 7:1; Deuteronomy 20:17; Joshua 24:18; etc). It was then confirmed to them by the covenant with David (2 Samuel 7:12-16) as expanded on in Psalms 2; etc. Thus they were looking for a Day of Blessing from YHWH. Amos was warning them that they must rather expect a Day of Woe. This was not, of course denying that one day YHWH would bring a Day of Blessing on His people, as Amos 9:11-15 makes clear. It was rather emphasising that that Day could only come on those whose hearts were prepared. It could not come on a people living in open sinfulness.