On the Shabbat before Purim, the holiday on which we celebrate the foiling of Haman the Amalekite's plot to destroy the Jewish people, the weekly Torah reading is supplemented with the Zachor ("Remember!") 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.
When the Israelites left Egypt, no nation dared pick a fight with them. Who would start up with a people whose G‑d just smacked around the mighty Egypt with ten awesome plagues, and drowned the surviving few in the sea? Only Amalek, driven by profound hatred which defied logic, came to wage battle.
According to many Halachic authorities, there is a Biblical requirement for all men to hear the special Zachor reading.
The special Zachor haftorah (I Samuel 15:2-34) discusses G‑d's command to King Saul to destroy the people of Amalek.
"Parshat Zachor" is the second of four special readings added during or immediately before the month of Adar (the other three being Shekalim, Parah and Hachodesh).
Click here for the Zachor Reading along with commentary and insights; click here for more on Amalek.