בְּי֣וֹם אֶחָ֔ד בְּכָל־מְדִינ֖וֹת הַמֶּ֣לֶךְ אֲחַשְׁוֵר֑וֹשׁ בִּשְׁלוֹשָׁ֥ה עָשָׂ֛ר לְחֹ֥דֶשׁ שְׁנֵים־עָשָׂ֖ר הוּא־חֹ֥דֶשׁ אֲדָֽר׃
upon one day in all the provinces of king Ahasuerus, namely, upon the thirteenth day of the twelfth month, which is the month Adar.
Read halakhic rulings (Jewish law) on Esther 8:12: and classical halakhic sources.