הלכות מגילה א:א-ד

(1) It is a positive mitzvah ordained by the Rabbis to read the Megillah at the appointed time. It is well-known that this was ordained by the Prophets.Everyone is obligated in this reading: men, women, converts, and freed slaves. Children should also be trained to read it. Even the priests should neglect their service in the Temple and come to hear the reading of the Megillah.Similarly, Torah study should be neglected to hear the reading of the Megillah. Surely, this applies to the other mitzvot of the Torah: the observance of all of them is superseded by the reading of the Megillah. There is nothing that takes priority over the reading of the Megillah except the burial of a meit mitzvah - a corpse that has no one to take care of it. A person who encounters such a corpse should bury it and then read the Megillah.

(2) One can fulfill one's obligation by reading or by listening to another person's reading, provided one listens to a person who is obligated to hear the reading. For this reason, if the reader was a minor or mentally incompetent, one who hears his reading does not fulfill his obligation.

(3) It is a mitzvah to read the entire Megillah and to read it both at night and during the day. The entire night is an appropriate time for the night reading, and the entire day is appropriate for the day reading.Before the reading at night, one should recite three blessings. They are:Blessed are You, God, our Lord, King of the universe, who has sanctified us with His commandments and commanded us to read the Megillah.Blessed are You, God, our Lord, King of the universe, who wrought miracles for our ancestors in previous days at this season.Blessed are You, God, our Lord, King of the universe, who has granted us life, sustained us, and enabled us to reach this occasion.During the day, one should not recite the final blessing.In places where it is customary to recite a blessing after the reading, the following blessing should be recited:Blessed are You, God, our Lord, King of the universe, who wages our battles and executes judgment on our behalf, who avenges the vengeance wrought against us, who exacts retribution from our enemies on our behalf, and who acquits justly all our mortal enemies. Blessed are You, God, the Almighty, who exacts payment on behalf of His nation Israel from all their oppressors, the God of salvation.

(4) What is the appropriate time for the Megillah to be read? The Sages ordained many different times for its reading, as implied by Esther 9:31: "To confirm these days of Purim in their appointed times." The following are the days on which the Megillah is read:Every city, whether in Eretz Yisrael or in the diaspora, that was surrounded by a wall at the time of Joshua bin Nun should read the Megillah on the fifteenth of Adar. This applies even when a wall does not surround the city at present. Such a city is called a כרך.Every city that was not surrounded by a wall at the time of Joshua bin Nun should read the Megillah on the fourteenth of Adar. This applies even when there is a wall surrounding the city at present. Such a city is called an עיר.