With gratitude to the Source Sheet compiled by David Schwartz
Purim is a post-Biblical holiday in the month of Adar. It is outlined in the Megillat Esther - the Book of Esther, in the third section of the Hebrew Bible, the Tanach.
(כב) כַּיָּמִ֗ים אֲשֶׁר־נָ֨חוּ בָהֶ֤ם הַיְּהוּדִים֙ מֵאֹ֣יְבֵיהֶ֔ם וְהַחֹ֗דֶשׁ אֲשֶׁר֩ נֶהְפַּ֨ךְ לָהֶ֤ם מִיָּגוֹן֙ לְשִׂמְחָ֔ה וּמֵאֵ֖בֶל לְי֣וֹם ט֑וֹב לַעֲשׂ֣וֹת אוֹתָ֗ם יְמֵי֙ מִשְׁתֶּ֣ה וְשִׂמְחָ֔ה וּמִשְׁלֹ֤חַ מָנוֹת֙ אִ֣ישׁ לְרֵעֵ֔הוּ וּמַתָּנ֖וֹת לָֽאֶבְיֹנִֽים׃
(22) 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.
This is where we get the tradition of Mishloach Manot (gifts to friends), Matanot L'Evyonim (giving to the poor)
(כח) וְהַיָּמִ֣ים הָ֠אֵ֠לֶּה נִזְכָּרִ֨ים וְנַעֲשִׂ֜ים בְּכׇל־דּ֣וֹר וָד֗וֹר מִשְׁפָּחָה֙ וּמִשְׁפָּחָ֔ה מְדִינָ֥ה וּמְדִינָ֖ה וְעִ֣יר וָעִ֑יר וִימֵ֞י הַפּוּרִ֣ים הָאֵ֗לֶּה לֹ֤א יַֽעַבְרוּ֙ מִתּ֣וֹךְ הַיְּהוּדִ֔ים וְזִכְרָ֖ם לֹא־יָס֥וּף מִזַּרְעָֽם׃ {ס}
(28) Consequently, these days are recalled and observed in every generation: by every family, every province, and every city. And these days of Purim shall never cease among the Jews, and the memory of them shall never perish among their descendants.
The commandment to listen to Megillat Esther, so that for all time, Jews should remember and observe Purim, the story of Esther. The Shabbat before Purim is called Shabbat Zachor - the Shabbat of memory. We are told to blot out the memory of the Amalekite, but always to remember the Amalekite.
(יז) זָכ֕וֹר אֵ֛ת אֲשֶׁר־עָשָׂ֥ה לְךָ֖ עֲמָלֵ֑ק בַּדֶּ֖רֶךְ בְּצֵאתְכֶ֥ם מִמִּצְרָֽיִם׃ (יח) אֲשֶׁ֨ר קָֽרְךָ֜ בַּדֶּ֗רֶךְ וַיְזַנֵּ֤ב בְּךָ֙ כׇּל־הַנֶּחֱשָׁלִ֣ים אַֽחֲרֶ֔יךָ וְאַתָּ֖ה עָיֵ֣ף וְיָגֵ֑עַ וְלֹ֥א יָרֵ֖א אֱלֹהִֽים׃ (יט) וְהָיָ֡ה בְּהָנִ֣יחַ יהוה אֱלֹהֶ֣יךָ ׀ לְ֠ךָ֠ מִכׇּל־אֹ֨יְבֶ֜יךָ מִסָּבִ֗יב בָּאָ֙רֶץ֙ אֲשֶׁ֣ר יְהֹוָה־אֱ֠לֹהֶ֠יךָ נֹתֵ֨ן לְךָ֤ נַחֲלָה֙ לְרִשְׁתָּ֔הּ תִּמְחֶה֙ אֶת־זֵ֣כֶר עֲמָלֵ֔ק מִתַּ֖חַת הַשָּׁמָ֑יִם לֹ֖א תִּשְׁכָּֽח׃ {פ}
(17) Remember what Amalek did to you on your journey, after you left Egypt— (18) how, undeterred by fear of God, he surprised you on the march, when you were famished and weary, and cut down all the stragglers in your rear. (19) Therefore, when your God יהוה grants you safety from all your enemies around you, in the land that your God יהוה is giving you as a hereditary portion, you shall blot out the memory of Amalek from under heaven. Do not forget!
Drinking for Purim!!
אָמַר רָבָא: מִיחַיַּיב אִינִישׁ לְבַסּוֹמֵי בְּפוּרַיָּא עַד דְּלָא יָדַע בֵּין אָרוּר הָמָן לְבָרוּךְ מָרְדֳּכַי. רַבָּה וְרַבִּי זֵירָא עֲבַדוּ סְעוּדַת פּוּרִים בַּהֲדֵי הֲדָדֵי. אִיבַּסּוּם. קָם רַבָּה שַׁחְטֵיהּ לְרַבִּי זֵירָא. לְמָחָר, בָּעֵי רַחֲמֵי וְאַחֲיֵיהּ. לְשָׁנָה, אֲמַר לֵיהּ: נֵיתֵי מָר וְנַעֲבֵיד סְעוּדַת פּוּרִים בַּהֲדֵי הֲדָדֵי. אֲמַר לֵיהּ: לָא בְּכֹל שַׁעְתָּא וְשַׁעְתָּא מִתְרְחִישׁ נִיסָּא.
Rava said: A person is obligated to become intoxicated with wine on Purim until he is so intoxicated that he does not know how to distinguish between cursed is Haman and blessed is Mordecai. The Gemara relates that Rabba and Rabbi Zeira prepared a Purim feast with each other, and they became intoxicated to the point that Rabba arose and slaughtered Rabbi Zeira. The next day, when he became sober and realized what he had done, Rabba asked God for mercy, and revived him. The next year, Rabba said to Rabbi Zeira: Let the Master come and let us prepare the Purim feast with each other. He said to him: Miracles do not happen each and every hour, and I do not want to undergo that experience again.
Wearing costumes - hiding ourselves. Esther hid who she was. God's not even mentioned in the story - God is hidden. Mordecai is robed in royal clothes, someone he wasn't. Cinderella lived between her poor, abusive life with her "family", and the royal princess, in a beautiful gown that would disappear.
(י) לֹא־הִגִּ֣ידָה אֶסְתֵּ֔ר אֶת־עַמָּ֖הּ וְאֶת־מֽוֹלַדְתָּ֑הּ כִּ֧י מׇרְדֳּכַ֛י צִוָּ֥ה עָלֶ֖יהָ אֲשֶׁ֥ר לֹא־תַגִּֽיד׃ {ס}
(10) Esther did not reveal her people or her kindred, for Mordecai had told her not to reveal it.
You think, what do you want?
You think, make a decision.
Why not stay and be caught?
You think, well, it's a thought,
What would be his response?
But then what if he knew
Who you were when you know
That you're not what he thinks
That he wants?
And then what if you are?
What a Prince would envision?
Although how can you know
Who you are till you know
What you want, which you don't?
So then which do you pick:
Where you're safe, out of sight,
And yourself, but where everything's wrong?
Or where everything's right
And you know that you'll never belong?