So Moses stretched out his hand toward heaven follows exactly the wording of verse 21. And there was thick darkness is literally “and there was darkness of darkness,” using an additional word for darkness that may suggest a supernatural or “eerie darkness” (Durham). Most translations simply intensify it as “deep darkness” (Jerusalem Bible), “pitch darkness” (Revised English Bible), or “total darkness” (Good News Translation). Three days implies three continuous twenty-four-hour periods of abnormal darkness. The translation should not suggest that this period included normal nighttime darkness.

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .