Joel 2:1-2 (ESV Strong's)
The Day of the Lord
1 Blow a trumpet in Zion;
sound an alarm on my holy mountain!
Let all the inhabitants of the land tremble,
for the day of the Lord is coming; it is near,
2 a day of darkness and gloom,
a day of clouds and thick darkness!
Like blackness there is spread upon the mountains
a great and powerful people;
their like has never been before,
nor will be again after them
through the years of all generations.
The day of the Lord is centered in Zion and its people. This is the great and final tribulation of a nation that rejected God. The curses they entered due to rebellion against the commandments of God will be spent and that cup emptied out.