Old Testament readings use the Septuagint , the Scripture the apostles quoted. Masoretic numbering shown for reference.Learn why
WEB

Esther 9:21

to enjoin them that they should keep the fourteenth and fifteenth days of the month Adar yearly,

Parallel Translations

LXXView canonical →
to establish these [as] joyful days, and to keep the fourteenth and fifteenth of Adar;
Brenton's Septuagint Translation
LXX2012View canonical →
to establish these [as] joyful days, and to keep the fourteenth and fifteenth of Adar;
Septuagint in American English (2012)
KJVView canonical →
To stablish this among them, that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly,
King James Version