לְקַיֵּם֮ עֲלֵיהֶם֒ לִהְי֣וֹת עֹשִׂ֗ים אֵ֠ת י֣וֹם אַרְבָּעָ֤ה עָשָׂר֙ לְחֹ֣דֶשׁ אֲדָ֔ר וְאֵ֛ת יוֹם־חֲמִשָּׁ֥ה עָשָׂ֖ר בּ֑וֹ בְּכָל־שָׁנָ֖ה וְשָׁנָֽה׃
to enjoin them that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly,
Read midrashic interpretation of Esther 9:21: and classical rabbinic exegesis.