Shulchan Aruch Chapter 686:  The Laws of the Fast of Esther (3 Chapters)

Note:  REMA in brackets [ ]

Introduction to this chapter:  The festival of Purim is a happy festival celebrating the miracle of Purim that occured around the early 500's BCE.  Haman, who was a decendant of Amalek (the same nation that Samuel the Prophet commandned King Saul to wipe out, but Saul erred and spared Aggag the King of Amalek) had a very prominant position in the inner circle of the Persion King Ataxerxis (some say Xerxis) and had a plot to kill all the Jews.  A miracle occured through Esther and Mordechai and this evil plan was thwarted.  Instead, Haman and his sons were hung and the Jews killed thousands of their enemies.  These historical events set the stage for the Jews subsequently returning to the land of Israel and rebuilding the Holy Temple.  Generally, Purim occurs on the 14th day of Adar and the day before is a fast called the Fast of Esther.  The Megillah of Esther is read during this holiday.

1.  On Hannukah and Purim it is permitted to fast before the holiday and after the holiday. 

2.  We fast (the fast of Esther) on the 13th day of the Hebrew month of Adar.  If the festival of Purim falls out on Sunday, the fast is moved to the Thursday before.  [This fast (of Esther) is not an abligation, therefore one may be lenient with it in a time of need.  For example:  A pregnant woman, nursing mother, or sick individual not dangerously ill, even if one has pain in the eyes that if it is a lot of discomfort, one should not fast but repay it at a later date.  However, other healthy individuals may not separate from the community.  In the case where the festival fell out on Sunday and was pushed to the Thursday before, where a Brit Milah (circumcision) also fell out on that day then it is permitted for those who attend the Brit Milah to eat and fast instead on Friday.]

3.  There are those who fast three days, in rememberance of the (original) Fast of Esther. 

Shulchan Aruch Chapter 687:  The Law of Who is Obligated to Read the Megillah (2 Chapters)
Note:  REMA in brackets [ ]

1.  One is obligated to read the Megillah on the night (after the Fast of Esther) and to reread it the next day.  With regard to the night reading, the time it can be read is all night long.  With regard to the day reading, the time it can be read is all day from sunrise (Naitz Ha'chamah) until the end of the day.  If he read it at first light (Amud Ha'sha'char) then he does fullfill his obligation. [1]

2.  You must disrupt learing Torah to hear the Megillah, and surely other Torah commandemnts as they are all pushed aside to hear the Megillah.  There is nothing that pushes the reading of the Megillah off except for a Mait Mitzvah (a found unburied and unclaimed corpse) that has no one to bury it [according to its need] in that one should bury it and afterwards read the Megillah.  [The above is only relavent where there is time to do them both, but if there is no time to do them both, then no Biblical Comandment can be pushed asside to read the Megillah.  Where the unclaimed corpse predominates, that (only stops the reading) is if it is impossible to read the Megillah afterwards. 

1.  Naitz Ha'chamah is sunrise, where the sun starts to be visible at sea level above the horizon.  Amud Hashachar or first light is either 72 or 90 minutes before sunrise, depending on the authority.
Translated by Dr. Jay Dinovitser 01/2013