When is Purim usually celebrated?
Regular Purim - For most people, Purim is celebrated on the 14th day of Adar. Megillat Esther (the book of Esther) relates that the Jews fought against their enemies on the 13th of Adar and rested and celebrated on the 14th.
Shushan Purim - In walled cities, such as Jerusalem, Purim is celebrated on the 15th of Adar. According to Megillat Esther, the Jews who lived in the walled city of Shushan were still fighting for their lives on the 14th and couldn't celebrate until the 15th. The rabbis instituted that Jews living in cities that had a wall around them in the time of Joshua would celebrate Shushan Purim.
וּשְׁאָ֣ר הַיְּהוּדִ֡ים אֲשֶׁר֩ בִּמְדִינ֨וֹת הַמֶּ֜לֶךְ נִקְהֲל֣וּ ׀ וְעָמֹ֣ד עַל־נַפְשָׁ֗ם וְנ֙וֹחַ֙ מֵאֹ֣יְבֵיהֶ֔ם ... בְּיוֹם־שְׁלֹשָׁ֥ה עָשָׂ֖ר לְחֹ֣דֶשׁ אֲדָ֑ר וְנ֗וֹחַ בְּאַרְבָּעָ֤ה עָשָׂר֙ בּ֔וֹ וְעָשֹׂ֣ה אֹת֔וֹ י֖וֹם מִשְׁתֶּ֥ה וְשִׂמְחָֽה׃ והיהודיים [וְהַיְּהוּדִ֣ים] אֲשֶׁר־בְּשׁוּשָׁ֗ן נִקְהֲלוּ֙ בִּשְׁלֹשָׁ֤ה עָשָׂר֙ בּ֔וֹ וּבְאַרְבָּעָ֥ה עָשָׂ֖ר בּ֑וֹ וְנ֗וֹחַ בַּחֲמִשָּׁ֤ה עָשָׂר֙ בּ֔וֹ וְעָשֹׂ֣ה אֹת֔וֹ י֖וֹם מִשְׁתֶּ֥ה וְשִׂמְחָֽה׃
The rest of the Jews, those in the king’s provinces, likewise mustered and fought for their lives. ... That was on the thirteenth day of the month of Adar; and they rested on the fourteenth day and made it a day of feasting and merrymaking. But the Jews in Shushan mustered on both the thirteenth and fourteenth days, and so rested on the fifteenth, and made it a day of feasting and merrymaking.
What are the mitzvot of Purim and when are they usually performed?
There are four mitzvot associated with Purim. A helpful way to remember the mitzvot is to remember them by their Hebrew names which all begin with the letter "mem".
(א) קְרִיאַת הַמְּגִלָּה בִּזְמַנָּהּ מִצְוַת עֲשֵׂה מִדִּבְרֵי סוֹפְרִים. וְהַדְּבָרִים יְדוּעִים שֶׁהִיא תַּקָּנַת הַנְּבִיאִים. וְהַכּל חַיָּבִים בִּקְרִיאָתָהּ אֲנָשִׁים וְנָשִׁים וְגֵרִים וַעֲבָדִים מְשֻׁחְרָרִים...
(1) We are bidden by the sages to read the Megillah at its proper time. It is widely known that this reading was prescribed by the prophets. Everyone is required to hear its reading: men, women, proselytes, and emancipated slaves...
(ג) מִצְוָה לִקְרוֹת אֶת כֻּלָּהּ. וּמִצְוָה לִקְרוֹתָהּ בַּלַּיְלָה וּבַיּוֹם. וְכָל הַלַּיְלָה כָּשֵׁר לִקְרִיאַת הַלַּיְלָה. וְכָל הַיּוֹם כָּשֵׁר לִקְרִיאַת הַיּוֹם. וּמְבָרֵךְ קֹדֶם קְרִיאָתָהּ בַּלַּיְלָה שָׁלֹשׁ בְּרָכוֹת ....
(3) It is commandment to read all of it; and it is a commandment to read it at night and during the day. And all of the night is fit for the reading of the night; and all of the day is fit for the reading of the day. And before its reading at night, one recites three blessings
(טו)...וְכֵן חַיָּב אָדָם לִשְׁלֹחַ שְׁתֵּי מְנוֹת בָּשָׂר אוֹ שְׁנֵי מִינֵי תַּבְשִׁיל אוֹ שְׁנֵי מִינֵי אֳכָלִין לַחֲבֵרוֹ שֶׁנֶּאֱמַר (אסתר ט יט) "וּמִשְׁלוֹחַ מָנוֹת אִישׁ לְרֵעֵהוּ", שְׁתֵּי מָנוֹת לְאִישׁ אֶחָד. וְכָל הַמַּרְבֶּה לִשְׁלֹחַ לְרֵעִים מְשֻׁבָּח. וְאִם אֵין לוֹ מַחֲלִיף עִם חֲבֵרוֹ זֶה שׁוֹלֵחַ לְזֶה סְעֵדָּתוֹ וְזֶה שׁוֹלֵחַ לְזֶה סְעֵדָּתוֹ כְּדֵי לְקַיֵּם וּמִשְׁלוֹחַ מָנוֹת אִישׁ לְרֵעֵהוּ:
(15) ...And likewise is a person obligated to send two portions of meat or two types of dishes or two types of food to his fellow, as it is stated (Esther 9:22) "and sending portions, one man to another" — two portions to one man. And anyone who increases sending to friends is praiseworthy. And if he has no [means to do this], he should trade with his fellow: This one sends his meal to that one and that one sends his meal to this one — in order to fulfill, "and sending portions, one man to another."
3. Mishteh (משתה) - The Purim feast. Eating, drinking and merrymaking are the hallmarks of the Purim feast. The Purim feast takes place on the day of Purim. As long as it begins while it is still Purim, the feast can last into the night.
(טו) כֵּיצַד חוֹבַת סְעֵדָּה זוֹ. שֶׁיֹּאכַל בָּשָׂר וִיתַקֵּן סְעֵדָּה נָאָה כְּפִי אֲשֶׁר תִּמְצָא יָדוֹ. וְשׁוֹתֶה יַיִן עַד שֶׁיִּשְׁתַּכֵּר וְיֵרָדֵם בְּשִׁכְרוּתוֹ. ...
(15) How is the obligation of this meal? That one eat meat and prepare as pleasing a meal as his hand can [afford]. And he should drink wine until he becomes intoxicated and falls asleep from his intoxication...
4. Matanot L'evyonim (מתנות לאביונים) - Gifts to the poor. Money is given to at least two people in order to help them to buy food to celebrate on Purim. Giving early in the day of Purim is recommended so that the money can be used for the Purim feast.
(טז) וְחַיָּב לְחַלֵּק לָעֲנִיִּים בְּיוֹם הַפּוּרִים. אֵין פּוֹחֲתִין מִשְּׁנֵי עֲנִיִּים ... וְאֵין מְדַקְדְּקִין בִּמְעוֹת פּוּרִים אֶלָּא כָּל הַפּוֹשֵׁט יָדוֹ לִטּל נוֹתְנִין לוֹ. וְאֵין מְשַׁנִּין מָעוֹת פּוּרִים לִצְדָקָה אַחֶרֶת:
(יז) מוּטָב לָאָדָם לְהַרְבּוֹת בְּמַתְּנוֹת אֶבְיוֹנִים מִלְּהַרְבּוֹת בִּסְעֻדָּתוֹ וּבְשִׁלּוּחַ מָנוֹת לְרֵעָיו. שֶׁאֵין שָׁם שִׂמְחָה גְּדוֹלָה וּמְפֹאָרָה אֶלָּא לְשַׂמֵּחַ לֵב עֲנִיִּים וִיתוֹמִים וְאַלְמָנוֹת וְגֵרִים. שֶׁהַמְשַׂמֵּחַ לֵב הָאֻמְלָלִים הָאֵלּוּ דּוֹמֶה לַשְּׁכִינָה שֶׁנֶּאֱמַר (ישעיה נז טו) "לְהַחֲיוֹת רוּחַ שְׁפָלִים וּלְהַחֲיוֹת לֵב נִדְכָּאִים":
(16) One is required to distribute charity to the poor on Purim. We do not [give to] less than two poor people...The applicants for Purim money should not be scrutinized; it should be given to anyone who holds out his hand. Purim money must not be diverted to any other charity.
(17) One should rather spend more money on gifts to the poor than on his Purim banquet and presents to his friends. No joy is greater and more glorious than the joy of gladdening the hearts of the poor, the orphans, the widows, and the strangers. He who gladdens the heart of these unhappy people imitates God, as it is written: "I am … to revive the spirit of the humble, and to put heart into the crushed" (Isaiah 57:15).
What happens when Shushan Purim falls on Shabbat? When regular Purim falls on Friday, Shushan Purim would seem to be celebrated on Shabbat. This poses a problem since the mitzvot of Purim are not performed on Shabbat. So what is there to do?
This special occurrence is called Purim Meshulash or the three-fold Purim. When this occurs, the mitzvot are spread over three days.
Megillah - The megillah is read on the 14th of Adar, Friday, the same as those celebrating regular Purim. The two readings are on Thursday night and Friday morning.
Matanot L'evyonim - Tzedakah is given on Friday, the 14th of Adar. Since the money is used for providing for the Purim feast, it makes sense to give it early.
Al Hanisim - The special prayer that is added to birkat hamazon (blessing after meals) and the amidah is said on Shabbat, the 15th of Adar.
עַל הַנִּסִּים וְעַל הַפֻּרְקָן וְעַל הַגְּבוּרוֹת וְעַל הַתְּשׁוּעוֹת וְעַל הַמִּלְחָמוֹת שֶׁעָשִׂיתָ לַאֲבוֹתֵינוּ בַּיָּמִים הָהֵם בַּזְּמַן הַזֶּה.
We thank thee also for the miracles, for the redemption, for the mighty deeds and saving acts, wrought by thee, as well as for the wars which thou didst wage for our fathers in days of old, at this season.
Mishloach manot - Mishloach manot are given on the 16th of Adar, Sunday.
Mishteh - The Purim feast takes place on Sunday, the 16th of Adar.