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.)
The Rebbe describes the following as a core teaching of the Baal Shem Tov:
Not only are the pirouettes of a leaf as it falls off a tree,
the quivering of a blade of grass in the wind,
and all the details of the world around you
all brought into being, given life and directed every moment from Above—
but beyond that:
Every nuance is an essential component of a grand and G‑dly scheme, the gestalt of all those vital minutiae.
Every moment burns with the pulse of G‑d’s desire.
Meditate on this. And then think:
How much more so the details of my daily life.