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>> “4 But you, brethren, are not in darkness, so that this Day should overtake you as a thief.” <<
The Thessalonians knew that Yeshua would return to resurrect his bride at the Day of Trumpets, 42 months after the making known of the Man of Sin.
The Thessalonians, like the apostles, kept Yehova’s appointed times, and thus were fully aware of the particular day of the year on which he had to come. The day that no man knew of before it showed up in the sunset, Tishri 1.
OK. Luckily all of this will be resolved quickly. Sort of like the Presidential Election. ;-)