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
In cities that are surrounded by a wall dating from the days of Joshua (13th century BCE) -- a prominent example is the city of Jerusalem -- the festival of Purim is observed on the 15th of Adar (instead of the 14th), in commemoration of the fact that in the ancient walled city of Shushan, the first Purim was celebrated on this day (see "Today in Jewish History").
(For an overview of the Purim observances and links to more information, see "Laws and Customs" for Adar 14.)
“Today” (appearing 336 times in Moses’ words in Deuteronomy)
And what happens if I keep these mitzvahs because of yesterday? Because they are the traditions of my forefathers, and therefore I cherish them?
Or if I keep these mitzvahs because of tomorrow—so that I will achieve spiritual heights, arrive after this life in a lofty Garden of Eden, revisit this world renewed as the World To Come?
Then I am a blind fool, a willing captive of a passing dream. A sheep that surrenders in silence as they shear its wool.
In a mitzvah right now, I hold the ultimate experience of truth, of life, of G‑d Himself in my hands—and I look to the past or to the future, to what I can get out of this?!