And you shall observe means to “keep” (Good News Translation) or “celebrate” (New International Version). The basic meaning of the word is to watch or guard. It is used again in verse 24. The feast of unleavened bread is just one word, matsoth, meaning “unleavened cakes” (see the comment on verse 8), but here it stands for the seven-day feast or “festival” (Good News Translation) itself. New English Bible has “these commandments,” following the src Samaritan Pentateuchsrc* (mitsoth), but Revised English Bible has returned to matsoth in the src Masoretic textsrc*. Therefore it is better to translate “You shall celebrate the Festival of Unleavened Bread,” or “… Festival of Thin Bread” (Contemporary English Version), or “… Festival of Bread without Yeast.” (See the comment under the section heading before verse 14.)

For on this very day refers to the “first day” in verse 15, which is also “this day” in verse 14, the 15th of Abib. I brought … out—Yahweh is still speaking—is literally “I caused to go out.” The verb speaks of the action as already completed. Your hosts is a military term referring to a multitude, or a large number. Some translations try to keep the military aspect: “companies” (New Revised Standard Version), “ranks” (New Jewish Publication Society’s Tanakh, New American Bible), and “divisions” (New International Version). But more likely the Israelites were organized by “tribes” (Good News Translation, Contemporary English Version) rather than as “armies” (Jerusalem Bible). (Revised English Bible and New English Bible have “your tribal hosts.”) (See the comment on “tribe” at 2.1.)

Therefore is the usual conjunction waw, which may be omitted. You shall observe this day repeats the same verb as in the first clause, and this day is the same day referred to earlier. Throughout your generations means “from generation to generation” (Revised English Bible), or simply “For all time to come.” (See verse 14.) As an ordinance for ever is also the same expression used in verse 14.

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 .