The term likely sends our thoughts to the Last Day when Christ returns to the earth to judge the living and the dead. The combination “great” and “day” in the context of divine judgment, however, appears elsewhere in Scripture to describe days/periods of intense divine judgment without implying that the last day/judgment is necessarily in view (Jeremiah 30:7; Joel 2:11, Joel 2:31; Zephaniah 1:14; Malachi 4:5; notice that these prophets spoke of varying historical contexts and periods). With this Old Testament information in mind, there is no need to place the events of Revelation in general or of Revelation 16:1–21 in particular in the narrow slice of time just before Christ returns, or even press its details into the Last Day. As the outpouring of the bowls is an ongoing event (see on Revelation 16:1), so their consequences are not limited to one-time occasions but repeat themselves in various ways throughout the new dispensation.
14 For they are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty.