משנכנס אדר מרבין בשמחה
משנכנס אב ממעטין בשמחה כו' אמר רב יהודה בריה דרב שמואל בר שילת משמיה דרב כשם שמשנכנס אב ממעטין בשמחה כך משנכנס אדר מרבין בשמחה
§ The mishna teaches that from when the month of Av begins, one decreases acts of rejoicing. Rav Yehuda, son of Rav Shmuel bar Sheilat, said in the name of Rav: Just as when Av begins one decreases rejoicing, so too when the month of Adar begins, one increases rejoicing.
משנכנס אדר - ימי נסים היו לישראל פורים ופסח:
Whoever enters Adar: Days of miracles - these were for the Jews - Purim and Pesach

ויִּכְתֹּ֣ב מָרְדֳּכַ֔י אֶת־הַדְּבָרִ֖ים הָאֵ֑לֶּה וַיִּשְׁלַ֨ח סְפָרִ֜ים אֶל־כָּל־הַיְּהוּדִ֗ים אֲשֶׁר֙ בְּכָל־מְדִינוֹת֙ הַמֶּ֣לֶךְ אֲחַשְׁוֵר֔וֹשׁ הַקְּרוֹבִ֖ים וְהָרְחוֹקִֽים׃ לְקַיֵּם֮ עֲלֵיהֶם֒ לִהְי֣וֹת עֹשִׂ֗ים אֵ֠ת י֣וֹם אַרְבָּעָ֤ה עָשָׂר֙ לְחֹ֣דֶשׁ אֲדָ֔ר וְאֵ֛ת יוֹם־חֲמִשָּׁ֥ה עָשָׂ֖ר בּ֑וֹ בְּכָל־שָׁנָ֖ה וְשָׁנָֽה׃ כַּיָּמִ֗ים אֲשֶׁר־נָ֨חוּ בָהֶ֤ם הַיְּהוּדִים֙ מֵא֣וֹיְבֵיהֶ֔ם וְהַחֹ֗דֶשׁ אֲשֶׁר֩ נֶהְפַּ֨ךְ לָהֶ֤ם מִיָּגוֹן֙ לְשִׂמְחָ֔ה וּמֵאֵ֖בֶל לְי֣וֹם ט֑וֹב לַעֲשׂ֣וֹת אוֹתָ֗ם יְמֵי֙ מִשְׁתֶּ֣ה וְשִׂמְחָ֔ה וּמִשְׁל֤וֹחַ מָנוֹת֙ אִ֣ישׁ לְרֵעֵ֔הוּ וּמַתָּנ֖וֹת לָֽאֶבְיוֹנִֽים׃

Mordecai recorded these events. And he sent dispatches to all the Jews throughout the provinces of King Ahasuerus, near and far, charging them to observe the fourteenth and fifteenth days of Adar, every year— the same days on which the Jews enjoyed relief from their foes and the same month which had been transformed for them from one of grief and mourning to one of festive joy. They were to observe them as days of feasting and merrymaking, and as an occasion for sending gifts to one another and presents to the poor.

ארבעים ושמונה נביאים ושבע נביאות נתנבאו להם לישראל ולא פחתו ולא הותירו על מה שכתוב בתורה חוץ ממקרא מגילה מאי דרוש אמר רבי חייא בר אבין אמר רבי יהושע בן קרחה ומה מעבדות לחירות אמרי' שירה ממיתה לחיים לא כל שכן?... רב נחמן אמר קרייתא זו הלילא רבא אמר בשלמא התם (תהלים קיג, א) הללו עבדי ה' ולא עבדי פרעה אלא הכא הללו עבדי ה' ולא עבדי אחשורוש אכתי עבדי אחשורוש אנן

The Sages taught in a baraita: Forty-eight prophets and seven prophetesses prophesied on behalf of the Jewish people, and they neither subtracted from nor added onto what is written in the Torah, introducing no changes or additions to the mitzvot except for the reading of the Megilla, which they added as an obligation for all future generations. The Gemara asks: What exposition led them to determine that this was a proper mode of action? On what basis did they add this mitzva? Rabbi Ḥiyya bar Avin said that Rabbi Yehoshua ben Korḥa said that they reasoned as follows: If, when recalling the exodus from Egypt, in which the Jews were delivered from slavery to freedom, we recite songs of praise, the Song of the Sea and the hymns of hallel, then, in order to properly recall the miracle of Purim and commemorate God’s delivering us from death to life, is it not all the more so the case that we must sing God’s praise by reading the story in the Megilla? The Gemara asks: If so, our obligation should be at least as great as when we recall the exodus from Egypt, and let us also recite hallel on Purim. The Gemara answers: Hallel is not said on Purim, because hallel is not recited on a miracle that occurred outside Eretz Yisrael. The Gemara asks: If so, with regard to the exodus from Egypt as well, which was a miracle that occurred outside Eretz Yisrael, how are we able to recite songs of praise? The Gemara answers: As it is taught in a baraita: Prior to the time when the Jewish people entered Eretz Yisrael, all lands were deemed fit for songs of praise to be recited for miracles performed within their borders, as all lands were treated equally. But after the Jewish people entered Eretz Yisrael, that land became endowed with greater sanctity, and all the other lands were no longer deemed fit for songs of praise to be recited for miracles performed within them. Rav Naḥman said an alternative answer as to why hallel is not recited on Purim: The reading of the Megilla itself is an act of reciting hallel. Rava said a third reason why hallel is not recited on Purim: Granted that hallel is said there, when recalling the exodus from Egypt, as after the salvation there, they could recite the phrase in hallel: “Give praise, O servants of the Lord” (Psalms 113:1); after their servitude to Pharaoh ended with their salvation, they were truly servants of the Lord and not servants of Pharaoh. But can it be said here, after the limited salvation commemorated on Purim: “Give praise, O servants of the Lord,” which would indicate that after the salvation the Jewish people were only servants of the Lord and not servants of Ahasuerus? No, even after the miracle of Purim, we were still the servants of Ahasuerus, as the Jews remained in exile under Persian rule, and consequently the salvation, which was incomplete, did not merit an obligation to say hallel.

The Therapeutic Joy of Purim (Purim 5775)- Rabbi Jonathan Sacks

Why an entire month? The key events were focused on a few days, the thirteenth to the fifteenth, not the whole month. And why simcha? We can understand why the Jews of the time felt exhilaration. The decree sentencing them to death had been rescinded. Their enemies had been punished. Haman had been hanged on the very gallows he had prepared for Mordechai. Mordechai himself had been raised to greatness.

But is joy the emotion we should feel in perpetuity, remembering those events?...Surely what we should feel is relief, not joy. Pesach is the proof. The word “joy” is never mentioned in the Torah in connection with it.

Besides which, the Talmud asks why we do not say Hallel on Purim. It gives several answers. The most powerful is that in Hallel we say, “Servants of the Lord, give praise,” – meaning that we are no longer the servants of Pharaoh. But, says the Talmud, even after the deliverance of Purim, Jews were still the servants of Achashverosh (Megillah 14a). Tragedy had been averted but there was no real change in the hazards of life in the Diaspora.

It seems to me therefore that the simcha we celebrate throughout the month of Adar is different from the normal joy we feel when something good and positive has happened to us or our people. That is expressive joy. The simcha of Adar, by contrast, is therapeutic joy.

Imagine what it is to be part of a people that had once heard the command issued against them: “to destroy, kill and annihilate all the Jews—young and old, women and children—on a single day” (Est. 3:13). We who live after the Holocaust, who have met survivors, heard their testimony, seen the photographs and documentaries and memorials, know the answer to that question. On Purim the Final Solution was averted. But it had been pronounced. Ever afterward, Jews knew their vulnerability. The very existence of Purim in our historical memory is traumatic.

The Jewish response to trauma is counterintuitive and extraordinary. You defeat fear by joy. You conquer terror by collective celebration. You prepare a festive meal, invite guests, give gifts to friends. While the story is being told, you make a rumbustious noise as if not only to blot out the memory of Amalek, but to make a joke out of the whole episode. You wear masks. You drink a little too much. You make a Purim spiel.

Precisely because the threat was so serious, you refuse to be serious – and in that refusal you are doing something very serious indeed. You are denying your enemies a victory. You are declaring that you will not be intimidated. As the date of the scheduled destruction approaches, you surround yourself with the single most effective antidote to fear: joy in life itself. As the three-sentence summary of Jewish history puts it: “They tried to destroy us. We survived. Let’s eat.”