The words וָיוֹם יוֹם are written twice in the megillah. It states (Esther 2:11) מ ָרְדֳּכַי וָיוֹם יוֹם וּבְכָל יֵּעָשֶׂה וּמַה אֶסְת ֵּר ש ְׁלוֹם אֶת לָדַעַת ה ַנָּשִׁים בֵּית חֲצַר לִפ ְנֵי מִתְה ַ לֵּ ךְּבָּה, "Day after day Mordechai would walk about in front of the courtyard... to learn about Ester's well-being and what would become of her." The Sfas Emes (5637, ובכל ה"ד) writes, "This is certainly a great deed. For three or four years, from the time Esther was taken to Achashveirosh, Mordechai HaTzaddik would go every day to check up on Esther, since she was an orphan, and she was suffering in captivity in the house of a goy. In this merit, the miracle occurred." The Purim miracle came about through Mordechai's devotion and caring to help Esther. We learn from this the greatness of doing deeds of chesed. It was in this merit that Hashem saved us, in the days of Mordechai and Esther.
It also states וָיוֹם יוֹם in the following verses (3:2-4): וּמִשְׁתַּח ֲוִים כֹּרְע ִים הַמֶּלֶ ךְ ַׁעַרבְּש ֶׁראֲש הַמֶּלֶ ךְ ע ַבְדֵי וְכָל וַיֹּאמ ְ רוּ ,יִשְׁתַּח ֲוֶה וְלֹא יִכְרַע לֹא וּמ ָרְדֳּכַי הַמֶּלֶ ךְ לוֹ צִוָּה כֵן כִּי לְהָמָן אֵת עוֹבֵר אַת ָּה ַמ ַדּוּע לְמ ָרְדֳּכָי הַמֶּלֶ ךְ ַׁעַרבְּש ֶׁראֲש ךְ הַמֶּל ֶ ע ַבְדֵי אֲל ֵיהֶם שָׁמַע וְלֹא וָיוֹם יוֹם אֵל ָיו כְּאָמְרָם וַיְהִי ,הַמֶּלֶ ךְ מ ִצְוַת, "All the king's servants at the king's gate would bow down and prostrate themselves before Haman, for so had the king commanded concerning him. But Mordechai would not bow and would not prostrate himself. The king's servants who were at the king's gate said to Mordechai, 'Why do you disobey the king's command?" Now it happened when they said this to him day after day and he did not heed them..."
The Sfas Emes explains that one good deed led to another. Since Mordechai came וָיוֹם יוֹם every day to check up on Esther, he would meet with Haman every day. The Sfas Emes writes, "Everything written in the megillah was needed for the miracle... By Mordechai going there daily, he merited to win over Haman HaRasha and to anger him every day."
A similar lesson is written by the Chasam Sofer (Drashos ד"טו ג"קכ ,ד אדר ח"ער). "When one thinks about the great miracle of Purim, its foundation is based on Mordechai's gemilus chasadim, that he raised Hadassah (who was Esther) when she was orphaned from both parents. She is called אסתר, because... she suffered פנים הסתר, that Hashem concealed His face from her. Mordechai, the greatest of the Sanhedrin, always sat at the king's gates (המלך שער). He wasn't concerned about his own honor. He took her as a daughter (לו מרדכי לקחה לבת, see Esther 2:7), and she was to him עינו כבבת, like the pupil of his eye. This is because after she was taken to Achashveirosh’s house, every morning and every evening, Mordechai would go to hear how she was faring. It also seems that the המלך שער alludes to the gates of the King of the world. Mordechai would pray for her by the gates of compassion, that she shouldn't be harmed and nothing bad should occur, chas v'shalom. In the merit of his chesed, they both became the דרחמנא שלוחי, Hashem's messengers to eradicate the cruel Amalek from the world and to build the second Beis HaMikdash."
It states (Tehillim 121:5) צלך 'ה, "Hashem is your shadow." The Midrash teaches that just as a shadow mimics what a person does, 'ה צלך means Hashem copies what people do. The Alshich HaKadosh (quoted in Arvei Nachal, Mikeitz, Drush 1, ואמנם ה"ד) teaches that Mordechai and Esther cared for one another, and they helped one another. This aroused Hashem to act accordingly, to care and have compassion for the Jewish nation.
The Alshich writes, "Mordechai didn't think about what he could gain, but rather what Esther could gain. When he heard Bigson and Seresh plotting to kill Achashverosh, he knew that whoever saved the king would receive a grand reward. Mordechai told Esther about the plot. [He wanted her to be rewarded.] But Esther wanted Mordechai to get the credit, so she told Achashverosh in the name of Mordechai. It states (Esther 2:22) לאסתר ויגד למרדכי הדבר ויודע מרדכי בשם למלך אסתר ותאמר המכלה, 'The matter became known to Mordechai, and he told it to Queen Esther, and Esther told the king in Mordecai's name.' They were both trying to help each other, and in the merit of their selfless caring for one another, Hashem saved them and the entire Jewish nation".
