This Shabbat we read the Torah portion called Ki Teitzei which contains the command to Remember what Amalek did:
http://www.chabad.org/parshah/torahreading.asp?AID=36237&p=7Devarim 2517. You shall remember what Amalek did to you on the way, when you went out of Egypt,
18. how he happened upon you on the way and cut off all the stragglers at your rear, when you were faint and weary, and he did not fear G-d.
19. [Therefore,] it will be, when the Lord your G-d grants you respite from all your enemies around [you] in the land which the Lord, your G-d, gives to you as an inheritance to possess, that
you shall obliterate the remembrance of Amalek from beneath the heavens.
You shall not forget!