(טו) וַתֹּ֥אמֶר אֶסְתֵּ֖ר לְהָשִׁ֥יב אֶֽל־מָרְדֳּכָֽי׃ (טז) לֵךְ֩ כְּנ֨וֹס אֶת־כָּל־הַיְּהוּדִ֜ים הַֽנִּמְצְאִ֣ים בְּשׁוּשָׁ֗ן וְצ֣וּמוּ עָ֠לַי וְאַל־תֹּאכְל֨וּ וְאַל־תִּשְׁתּ֜וּ שְׁלֹ֤שֶׁת יָמִים֙ לַ֣יְלָה וָי֔וֹם גַּם־אֲנִ֥י וְנַעֲרֹתַ֖י אָצ֣וּם כֵּ֑ן וּבְכֵ֞ן אָב֤וֹא אֶל־הַמֶּ֙לֶךְ֙ אֲשֶׁ֣ר לֹֽא־כַדָּ֔ת וְכַאֲשֶׁ֥ר אָבַ֖דְתִּי אָבָֽדְתִּי׃ (יז) וַֽיַּעֲבֹ֖ר מָרְדֳּכָ֑י וַיַּ֕עַשׂ כְּכֹ֛ל אֲשֶׁר־צִוְּתָ֥ה עָלָ֖יו אֶסְתֵּֽר׃
(15) Then Esther asked them to tell Mordecai: (16) ’Go, gather together all the Jews that are present in Shushan, and fast for me, and neither eat nor drink three days, night or day; I also and my maidens will also fast; and so will I go in to the king, which is not according to the law; and if I perish, I perish.’ (17) So Mordecai went his way, and did according to all that Esther had commanded him.
רמב"ם, הלכות תעניות 5:5
ונהגו כל ישראל בזמנים אלו להתענות בשלשה עשר באדר זכר לתענית שהתענו בימי המן שנאמר (אסתר ט, לא): דברי הצומות וזעקתם.
Rambam, Laws of Minor Fasts 5:5
And in those times all of Israel would undertake to fast on the 13th day of Adar in remembrance of the fast that Esther undertook in the days of Haman, as it says (Esther 9:31): "the matters of their fastings and their prayers."
Arthur Waskow, Seasons of Our Joy
During the day before Purim begins -- that is, from dawn to sundown on the 13th of Adar -- we observe the Fast of Esther. its timing accords not with that of Esther's own fast before she dares to explain to the King who she is, but with the dreadful day of fear and battle when groups of Jews and Haman's ganges were struggling for their lives in the towns and cities of Persia. So this day, too, underlines the anxiety and tension of the struggle to live under murderous oppression. Then at nightfall, comes release.