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").
All over the world, Tachanun is omitted due to the special joy of the day.
(For an overview of the Purim observances and links to more information, see "Laws and Customs" for Adar 14.)
The last written work of the Rebbe before his stroke centered on the following thought. The Rebbe personally handed a copy to thousands of people. I believe it is a summary of who we are and what we must do:
Self-sacrifice in a land of freedom penetrates to the bone.
I saw men and women who sacrificed all they had to defy the religious persecution of the Bolshevik regime. They came to a land of freedom and comfort—where is their greatness now?
Then there is the child of that land of freedom and comfort, worshipping it, chasing after it—but inside he is crushed by the spiritual void. His inner being does not let him alone, the spark inside that cries, “This is not what I truly want! I don’t want this world! I don’t want any worlds! All I want is Him alone!”
This is the crushing of an olive for its oil. The oil spreads and penetrates every fiber of his being. His every faculty begins to burn. And there shines the source of light that can never be extinguished nor dimmed.
It is the light from which the messianic era is formed.