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.” Good News Translation 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.”

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