As a nation, we were commanded to destroy the nation of Amalek. This nation came into existence at the same time as we did. Esav’s son Elifaz had a son Amalek. Esav and Elifaz’s legacy to Amalek was an undying hatred against the children of Yakov.
At the time of the exodus from Egypt, Amalek traveled hundreds of miles to ambush the newly freed nation in the hope of destroying them. We, as a nation, did not pose any threat to their sovereignty. They lived to the east of Canaan and were not among the Seven Nations occupying Eretz Yisroel. Nevertheless, their irrational hatred against Hashem and us compelled them to attack a harmless and seemingly defenseless nation. In the aftermath of their attack we were commanded to always remember the evil that is Amalek. It is the reading of this Parsha that is the fulfillment of this Biblical commandment. This mitzvah, according to most authorities, is not restricted by time and must be fulfilled by men and woman.
The Rabbi’s selected the Shabbos before Purim for the fulfillment of this Mitzvah because Haman was a direct descendent of Amalek, and Mordecai was a direct descendent of King Saul. The entire story of Purim is directly linked to this Mitzvah and the missed opportunity of King Saul that we read about in the Haftorah.