The festival of Purim celebrates the salvation of the Jewish people from Haman's plot "to destroy, kill and annihilate all the Jews, young and old, infants and women, in a single day."
The events of Purim extended over a period of several years, culminating in the victory celebrations of Adar 14-15 of 356 BCE. This year Purim begins Saturday at nightfall.
This being the Shabbat before Purim, on which we celebrate the foiling of Haman the Amalekite's plot to destroy the Jewish people, the weekly Parshah is supplemented with the Zachor reading (Deuteronomy 25:17-19) in which we are commanded to remember the evil of Amalek and to eradicate it from the face of the earth.
For the complete Purim guide, including Purim laws and customs,