In this verse God was confronting the nation of Judah. For they had been practicing religious festivals of their own design in direct rebellion against Him. In other words, they were taking God’s Word – adding and twisting it to fit their own desires and notions. Kinda sounds like religion today where God’s Word has been spun and misinterpreted to become more politically correct and not offensive anyone. But man’s job is not to reinterpret God’s Word in order to make everyone feel better. Man’s job simply is to obey what God has already written.
As defined, The Day of The Lord is a significant period of time described as a day of wrath, distress and anguish, trouble and ruin, darkness and gloom for all mankind. Isaiah 34: 2-8 describes The Day of the Lord as God’s judgment against the Nations…or His ultimate intervention in the affairs of His people. For in the Valley of decision, mankind won’t have the option to choose anymore.