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The Annual Purim Holiday: [1]

The Jews outside Shushan battled on the 13th and rested on the 14th while the Jews in Shushan battled on the 13th and 14th and rested on the 15th. Therefore it was enacted that the un-walled cities are to annually celebrate Purim on the 14th through rejoicing, feasting, and sending gifts to each other, and Matanos Laevyonim. This decree was written and sent to all the Jews in the kingdom of Achashveirosh. The holiday was called Purim after the lottery that was made by Haman.

The spread of the Holiday:[2]

By the first year, the annual Purim celebration was only instituted in Shushan. By the second year they instituted its celebration throughout all the cities of Jews. Esther sent an appeal to the Sages requesting them to authorize the Purim holiday as an everlasting Jewish day of celebration for all generations to come. At first the Sages were reluctant to grant her this request due to fear of reprisals from the gentile nations of future generations. She replied to them that the entire story is already recorded in the chronicles of Media and Persia. [The Sages then agreed to her request.] 

Why was the celebration of Purim set for the 14th and not the 13th?[3]

As the celebration was established based on the day that the survival of the Jewish people became apparent, which was the 14th of Adar. This is unlike Chanukah in which the celebration was established based on the successfulness of the war, which ended on the 25th of Kislev.