The single verse that is often cited for support for viewing the day of the Lord as a twenty-four hour day interpretation is: “But it shall be one day which shall be known to the LORD, not day, nor night: but it shall come to pass, that at evening time it shall be light” (Zech. 14:7 KJV).
However, the term “one day” should be better properly translated “unique day” or “never-ending day.” This is the rendering of most modern versions. For example: “And there shall be a unique day, which is known to the LORD, neither day nor night, but at evening time there shall be light” (Zech 14:7 ESV, emphasis mine). See also the NIV and NASB for the same rendering. In other words, translators recognize that the context suggests this new era will be unique in redemptive history. It will be the new established conditions that will be ongoing. Thus, rendering and interpreting it as “one day” misses the oracle’s point entirely.
In addition, the most authoritative Hebrew lexicon, The Hebrew and Aramaic Lexicon of the Old Testament, lists yôm (“day”) containing ten different meanings depending on its context. It is noteworthy that the lexicon does not include the instance of Zechariah 14:7 under the category of “day of twenty-four hours”; instead, it places it under the semantic rubric “day of Yahweh,” thereby suggesting a special sense of the word rather than a mere twenty-four hour day. Ludwig Koehler, Walter Baumgartner, and M.E.J. Richardson, eds., The Hebrew and Aramaic Lexicon of the Old Testament, Accordance electronic edition, version 3.0. (Leiden: Brill, 2000), 2:400. Moreover, the Hebrew term for “one/unique” is ʾehad, which, in this particular verse according to the lexicon, means “never-ending day.” Koehler, Baumgartner, and Richardson, HALOT, 1:30.
There is a last appeal to make this a twenty-four hour day by noting that the verse ends with, “in the evening there will be light.” The mention of “evening” is supposed to indicate the day is literal. The problem with this is the verse is located in a central section (vv. 6-9) figuratively describing the new and ongoing (not limited to a single day) conditions when the Lord will rule as king: “On that day there shall be no light, cold, or frost. And there shall be a unique day, which is known to the LORD, neither day nor night, but at evening time there shall be light. On that day living waters shall flow out from Jerusalem, half of them to the eastern sea and half of them to the western sea. It shall continue in summer as in winter. And the LORD will be king over all the earth. On that day the LORD will be one and his name one” (Zech. 14:6-9 ESV). The italics are mine, noting the figurative language describing the continuous nature of God’s kingdom. To argue here that the Day of the Lord is limited to a single day is untenable. Another verse that is sometimes appealed to is Isaiah 10:17: “The light of Israel will become a fire, their Holy One will become a flame; it will burn and consume the Assyrian king’s briers and his thorns in one day.” This verse is weak support, since it is not speaking of the universal Day of the Lord’s wrath; instead, it is specifically a judgment upon “the Assyrian king.” See Isaiah 10:5-19, especially v. 12, for this Assyrian context.