Positive commandment · Mitzvat Aseh #189 of 613
To constantly remember what Amalek did to us
לזכור מה שעשה לנו עמלק תדיר
The verse · Devarim 25:17
זָכוֹר אֵת אֲשֶׁר־עָשָׂה לְךָ עֲמָלֵק בַּדֶּרֶךְ בְּצֵאתְכֶם מִמִּצְרָיִם
“Remember what ῾Amaleq did to thee by the way, when you were come out of Miżrayim:”
The Rambam’s words
To constantly remember what Amalek did to us, as [Deuteronomy 25:17] states: "Remember what Amalek did to you."
לזכור מה שעשה לנו עמלק תדיר, שנאמר: "זכור, את אשר עשה לך עמלק" (דברים כה,יז).
Why · Sefer HaChinuch #603
To remember what Amalek did to us
“It is from the roots of this commandment [that it is] to put into our hearts that anyone who distresses Israel is despised in front of G-d, blessed be He; and that according to his evil and his great deceptive damage, will be his downfall and bad [occurrences], as you find with Amalek: That due to [the fact that] he committed a great evil to Israel — that he began to harm them — blessed be He, commanded us to “destroy his memory from the earth” and root it out after him, until its end. (See Sefer HaMitzvot, Positive Commandments 189.)”
Who & when
“This commandment is practiced in every place and at all times by males, because it is upon them to wage war to avenge the enemies — and not upon women.”
Where in the Mishneh Torah
Hilchot Kings and Wars הלכות מלכים ומלחמות