Halakhah for Berakhot 42:6

We do not know an established time in the year or the day for this remembrance with the heart and the mouth, such as we are commanded to remember the exodus from Egypt every day and every night. And the reason [for the latter] is that that remembrance is a fundamental principle of religion, and as we have spoken about this in detail in many other places in this book. But the reason for remembering what Amalek did, is only that the hatred in our hearts not be forgotten. And for this, it is enough to remember the matter once a year, or in two years, or in three years. And behold, in all the places of Israel, they read the Book of the Torah over a year, or two or three - at the very least. And behold, they fulfill this commandment with that. And perhaps we should say that the custom of Israel to read Parashat Zachor on a specific Shabbat each and every year is [as a law from the] Torah. And it is due to this commandment that they fixed it like that. And it is always the Shabbat before Purim. And the law should be to read it on Purim day, because it is from the matter of the day, since Haman the wicked was from the seed [of Amalek]; but in order to make known that before the miracle [of Purim], we were commanded in this remembrance, they fixed the [reading] before Purim. However they made it adjacent to Purim, in the way that they, may their memory be blessed, say (Berakhot 21b) in [certain] places, "they placed the matter adjacent to it."