The battles fought between the Jews and their enemies, which took place on Adar 13 throughout the Persian empire (see "Today in Jewish History" for that date), continued for two days -- Adar 13 and 14 -- in the capital city of Shushan, where there were a greater number of Jew haters. Thus the victory celebrations in Shushan were held on the 15th of Adar, and the observance of the festival of Purim was instituted for that day in Shushan and all walled cities. (See Laws and Customs below).
On this date, in the year following the Holy Temple’s destruction, G‑d tells Ezekiel to take up a lamentation for Pharaoh, king of Egypt, and for the Jewish nation’s other enemies, foretelling their ultimate downfall.
Read the prophecy here: Ezekiel ch. 32
Adar 15 is "Shushan Purim" -- the day on which Purim is observed in Jerusalem and in other ancient walled cities, in commemoration of the fact that in the ancient walled city of Shushan the original Purim celebration was on this date. (see above, Today in Jewish History).
When Shushan Purim falls on Shabbat -- as it does this years -- a unique phenomenon results: the "Triple" or Three-Day Purim (Purim Meshuleshet). Because a number of the Purim mitzvot cannot be performed on Shabbat, the observances are spread over a period of three days: the megillah reading and Giving to the Poor on Friday; Al Hanissim -- the special Purim addition to the daily prayers and Grace After Meals -- on Shabbat; and Sending Food Gifts to Friends and the Purim meal -- on Sunday. For the details of these laws, see summary and links in yesterday's Laws & Customs.
For more on the Three-Day Purim, click here.
(The Three-Day Purim phenomenon is unique to Shushan Purim, since the regular Purim -- Adar 14 -- cannot fall on Shabbat in the present-day configuration of Jewish calendar.)