When evening comes on: that is, “in the late afternoon,” “before sunset.”
When the sun is down: meaning after sunset, when the next day begins, according to the Hebrew reckoning.
Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .