In the first month is literally “in the first,” but it refers to the month of Abib (as in verse 2). On the fourteenth day corresponds to verse 6. At evening really places it on the fifteenth day, since the next day began at sunset (as discussed at verse 14). You shall eat unleavened bread corresponds to verse 8, the Passover meal. And so until the twenty-first day simply repeats the instruction given in verse 15.

At evening may present a problem if it is understood to include the evening of the twenty-first day, which was already the twenty-second day. That would be a total of eight evenings, which does not agree with the “seven days” emphasized again in the following verse. The preposition at also means “until” (New Revised Standard Version), “to the evening,” or “when the sun sets on the twenty-first day.” New English Bible and Revised English Bible insist on “the evening which begins the fourteenth day until the evening which begins the twenty-first day,” but this does not agree with the eating of the Passover meal, which was the evening following the slaying of the animal on the fourteenth day. (See verse 6 and the comment on verse 14). Another model is “From when the sun sets on the fourteenth day in the first month until it sets on the twenty-first day of that month, you must not eat….”

An alternative translation model for this verse is:

• You must refrain from eating bread made with yeast, beginning from the time the sun sets on the fourteenth day of the first month, until it sets on the twenty-first day of that same month.

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 .