Laws of Purim

1. Shabbat Clothing
It is proper to wear Shabbat and holiday clothing beginning in the evening.  One’s house should be clean and straightened up nicely to honor Purim (Rama, Orach Chaim 695:2).

2. Gifts to the Poor
One is required to give at least two gifts to two poor people, one gift to each poor person (Shulchan Aruch 694:1).  These gifts may only be given on Purim and not beforehand, during the day, and not at night. The basic law is that it is sufficient to give to each poor person a minimal amount, but the Rambam wrote: "It is preferable for a person to be liberal with his gifts to the poor than to be lavish in his festive meal or in his giving portions to his friend, because there is no greater and more splendid happiness than to gladden the hearts of the poor, the orphans, the widows and the converts. For one who gladdens the heart of the unfortunate is similar to the Divine Presence, as it says (Yeshayahu 57:15): To revive the spirit of the lowly and to revive the heart of the contrite" (Hilchot Megillah 2:17). It is possible, however, to take money for the additional gifts from one’s yearly Tzedakah obligation. We should not be particular with regards to whom we give Tzedakah on Purim, rather we give to anyone who extends his hand to receive. If one does not find poor people on Purim, he should set aside the money and give it to Tzedakah afterwards (Shulchan Aruch 694:1-2).

3. Festive Meal of Purim
One is required to eat a pleasant festive meal, such as with meat, bread and wine.  Women are also obligated in this. There is no obligation of a festive meal at night, but one should nevertheless increase his meal somewhat. It is also proper to rejoice somewhat at a festive meal on Shushan Purim (the next day - the 15th of Adar - when Purim is celebrated within a walled city). The majority of the festive meal of Purim must take place during the day. Even if one concludes it at night, one nevertheless recites "Al Ha-Nissim" in the Birkat Ha-Mazon (Rama, Orach Chaim 695:2).
It is a proper custom to eat the festive meal only after one fulfills the Mitzvah of sending portions to friends and gifts to the poor.  If the time of Minchah Gedolah (the earliest time to Daven the afternoon prayer - approximately midday) arrives - it is forbidden to begin the festive meal until one prays (Rama, Orach Chaim 695:2). One should engage somewhat in Torah learning before the festive meal (ibid.).

4. Drinking Wine at the Festive Meal of Purim
One is required to drink wine at the festive meal, but even if he does not drink, he nevertheless fulfills the obligation of having a festive meal on Purim (Mishnah Berurah in Biur Halachah 695:2). One is not obligated to become drunk but he should drink more than that to which he is accustomed. Thus he will fall asleep, and by sleeping he will not know the difference between cursing Haman and blessing Mordechai. The Meiri wrote that were are not obligated to become drunk and lower ourselves in the midst of rejoicing, "We are not commanded regarding joy of folly and nonsense but joy of pleasure which will bring out of it the love of Hashem, may He be Blessed, and thanksgiving of the miracles which He performed for us" (Megillah 7). Therefore, if one knows that he will disregard a Mitzvah (Netilat Yadayim, blessings, Birkat Ha-Mazon), or that he will not Daven Minchah or Ma’ariv, or that he will act frivolously – it is better for him not to get drunk.
Women are exempt from drinking wine.

5. Sending Portions to Friends
One is required to send at least two portions to one person.  Anyone who increases the amount given is praiseworthy, because on account of this love, fraternity, peace and friendship increase (Mishnah Berurah 695:4). We must send two types of food which may be consumed immediately, without needing to be cooked or prepared.  The sent items must be important and honorable in the eyes of the sender and the recipient. It may only be sent during the day and not at night. If one sent it earlier by mail, and it arrives on Purim, there are those who state that he has fulfilled his obligation (Shut Mahari Asad #207).
Women are also obligated in sending portions. Her husband may send it for her, but there are those who are strict that she should send for herself. Care should be taken that a woman sends to a woman, and a man sends to a man.
A boy is obligated in sending portions when he becomes 13 years old, and a girl when she become 12 years old. When a child arrives at the age of education (when they can learn proper conduct) it is proper to educate him to fulfill the Mitzvah.
Regarding one who sends to a minor under Bar Mitzvah - there are those who state that he does not fulfill his obligation. But a minor may send to a minor.
One may not send portions to someone who is mourning for his father or mother within twelve months, but it is permissible to send to his family members who are not mourners (Kitzur Shulchan Aruch 142:2).
Regarding one who send anonymously - there are those who state that he does not fulfill his obligation.  He should therefore send another one.
An agent - even though there are those who are careful to send it only by way of an agent, one may nevertheless bring it on his own or send it by way of a minor even though he is not included in the category of those who may be an "agent".

6. One who Forgot to Say: "Al Ha-Nissim"
If he forgot "Al Ha-Nissim" in the Shemonah Esrei or Birkat Ha-Mazon:
1. If he remembers before he says Hashem’s Name, he should return and recite "Al Ha-Nissim".
2. If he already said Hashem’s Name, he should continue.
But:
A. In the Shemonah Esrei, in "Elokai Netzor " before "Yehiyu Le-Ratzon," one should add: "We give thanks to you on the miracles...".
B. In the Birkat Ha-Mazon, before reciting "Ha-Rachaman Hu Yezakeinu", one should add: "May the Merciful One perform miracles and wonders for us as He performed for our ancestors in these days at this time in the days of..." (Rama, Orach Chaim 682:1).

7. When Adar Arrives, We increase in Joy
And we continue to rejoice until the end of the month of Nissan, or until the beginning of the mourning of Sefirat Ha-Omer.

8. Learning Torah

It is also proper to make one’s soul rejoice with additional Torah learning on this day, about which the Sages of Israel were zealous throughout the generations, as they wrote (Esther 8:16): "The Jews had light" - and "light" refers to "Torah."