On the first day refers to the 15th day of Abib, when the festival began. You shall hold is literally “there shall be” (so Revised English Bible). A holy assembly is a technical term for a religious gathering, or “a solemn assembly” (New Revised Standard Version). The basic meaning of assembly is a group of people who are called together. Note Good News Translation, “you are to meet for worship.” In some languages it will be necessary to have an explicit goal for the verb “worship”; for example, “You must meet together to worship me [Yahweh].” The seventh day is the last of the seven days of eating unleavened bread, with five days in between.
No work shall be done is literally “all labor will not be done.” Work is a general term that includes cooking, for it is specified as an exception in the next clause. This applies only to those days, that is, the first and the seventh days. In languages that do not use the passive voice, one may express this as “You must not do any work during these two days,” or even “Don’t do work of any kind during these two days.”
The word but is the same word ʾak used in verse 15, but here it has a restrictive function. It means “only” (New Revised Standard Version, New Jewish Publication Society’s Tanakh), or “except” (Revised English Bible, New International Version, New American Bible), and it introduces the one exception, in terms of what every one must eat. The idea of must is implied by the but (ʾak) and the only that follows. Therefore that only may be prepared by you allows for the necessary preparation of food, which is the only kind of work permitted on those two days. Good News Translation‘s “but you may prepare food for everyone” may seem too permissive. One may say “but you may prepare the necessary food for everyone.” It is possible to restructure the final part of this verse as follows: “The only work that I allow you to do during these two days is the preparing of food.”
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 .