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.)
Why is Torah compared to light? Because it tells us the place of each thing.
Because, in truth, there is no need to change the world. Everything is here.
Each thing has a place, and in that place it is good. Altogether, it is very good, a beautiful world. All that’s needed is a little light.
What is light? Light doesn't add anything or take away. It only reveals the meaning and purpose of all that it shines upon.
Think of your own home. In the dark, there is no way to know what belongs in your closet and what belongs in the laundry, what is ready for use and what is in need of repair. Instead, that which could be washed or repaired is rejected and despised, and your most valuable possessions may become the greatest hazards.
Switch on one little light and a dangerous place becomes a home. With every light you add, you become suddenly wealthier and more blessed.
So too, this world is meant to be G-d’s home. Torah is light. Shine it bright and heal the world.