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 month before Rosh Hashanah, taught Rabbi Schneur Zalman of Liadi, is a precious month. For thirty days, you can find the Ultimate King awaiting you outside the city, on a rustic path that winds through an empty meadow.
What’s the advantage of meeting the King in a meadow?
Because when you meet the King in His palace of the Days of Awe from Rosh Hashanah to Yom Kippur, much stands between you and Him—the carpet, the throne, the royal robes, the protocol, the language you must use, the awe and magnificence of the palace…
But in the field, the King wears blue jeans. And so do you. He even greets you with a smile, and expresses His delight that you came out to see Him.
Now it’s just you with the King.