This was on the thirteenth day refers to the events of verse 16.

They rested translates the same Hebrew verb translated in the preceding verse as “got relief.” Today’s English Version makes it explicit that the “relief” was from killing, but it may not be necessary to be this specific. There was no more danger, they were at peace, and they made that a day of feasting and gladness. It may be helpful to restate this as “they dedicated that day to feasting and joy” or “they made [or, cut] that day a day of joy and a day of feasting.”

Septuagint 9.17

The theme of “rest” is brought into sharp focus in the Greek version of this verse, which says both that they rested and that they made that same day a day of rest. Today’s English Version interprets the first rest to be respite from battle, while the second is that of celebration (so also Bible en français courant).

On the fourteenth day is literally “on the fourteenth day of the same month.”

The Septuagint omits the words “this was on the thirteenth day of the month of Adar,” since that information has already been given in verse 16 in the Septuagint.

Joy and gladness: see Septuagint 8.16, 17. The Greek says “they kept [or, observed/held] that day [a day] of rest with joy and celebration.”

Quoted with permission from Omanson, Roger L. and Noss, Philip A. A Handbook on the Book of Esther — Deuterocanon: The Greek Text. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .