The holiday of Purim celebrates G-d's saving of the Jewish people from the genocidal plot of Haman, the Persian king's prime minister.
It is celebrated on the 14th of the month of Adar. In walled cities, it is celebrated one day later, also known as Shushan Purim.
Learn more on Wikipedia's Purim article.
Besides purim questions, you may wish to see also questions with these somewhat related tags: mishloach-manot, megillat-esther, grapes-wine, inebriation, tractate-megillah, amalek, esther-the-woman, haman, parshat-zachor, four-parshiyot, purim-torah-in-jest