You is still singular in the Hebrew, although the Septuagint uses the plural here and in verse 7. Therefore is not in the Hebrew, but the verse begins with the conjunction waw, which allows for this. You shall … keep is the word meaning to watch, guard, or “observe” (New Jerusalem Bible). This ordinance uses the word chuqqah, which means “law” (New Jerusalem Bible), “statute” (Revised English Bible), “institution” (New Jewish Publication Society’s Tanakh), or “requirement” (Durham). Good News Translation and Contemporary English Version interpret it contextually as “Celebrate this festival,” and this will be a good model in many languages. (See the introduction to 20.22-26.)

At its appointed time refers to the “proper time” (New Revised Standard Version) or “scheduled time” (Durham) for observing the ordinance of unleavened bread, which has been specified in 12.18. (Contemporary English Version has “at the same time.”) The possessive pronoun its refers to the ordinance, or possibly to the festival itself, but it may be more natural to change its to “the,” since this is understood. From year to year means “each year,” or “year after year” (New International Version).

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 .