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.)
What is a mitzvah?
An act of uniting heaven and earth.
But heaven is spiritual and earth is material. To make spirit and matter one is impossible!
A creature of earth cannot make earth heaven. And a heavenly being cannot bring heaven to earth.
But for you, it is not impossible.
This mitzvah that I am commanding you today is not impossible for you. It’s not even distant. It’s very close to you, in your mouth and in your heart to do it. (Deut. 30:11)
It is close to you because the Torah was given to you both written and orally. (Rashi)
Because within you breathes the Creator of both heaven and earth.
So that when you speak the words of His written Torah, you bring heaven down to earth.
And when you explain the words of His oral Torah, you bring earth up to heaven.
Indeed, you accomplish the impossible every day.