“There are those that maintain that the reading of Parshat Zachor and Parshat Parah is a Torah obligation. Therefore, people living in an area in which there is not a congregation are obligated to come to a place that has a minyan for these Shabbatot. This is in order to hear these Torah readings that are Torah commandments.” (Shulchan Aruch, Orech Chaim 685:7)
The Shabbat prior to Purim, we read Parshat Zachor. This special reading is found at the end of Parshat Ki Tetze. It discusses two mitzvot. The first is the obligation to remember the evil of Amalek. The second is the obligation to destroy the very memory of this corrupt nation. Shulchan Aruch notes that, according to many authorities, the reading of Parshat Zachor is required in order to fulfill the mitzvah of remembering Amalek. Therefore, it is important for every person to hear this reading.
Parshat Zachor is one of two sections in the Torah that discusses the wickedness of Amalek. The second section is at the end of Parshat Beshalach. These passages describe the unprovoked war that Amalek waged against Bnai Yisrael. This section also records Hashem’s pledge to destroy Amalek. These passages are the Torah reading for Purim. Magen Avraham raises an interesting question. Can one fulfill the obligation to recall the wickedness of Amalek through the Purim Torah reading? This reading also discusses the wickedness of Amalek.
Magen Avraham suggests that one can fulfill the obligation to remember Amalek with the Purim reading. He argues that there is no reason for specifically requiring one to read the passages at the end of Parshat Ki Tetze. Neither is there any obvious reason for requiring that one fulfill the mitzvah the week before Purim.
Rav Yosef Dov Soloveitchik Zt”l disagrees. He points out that there is a basic difference between these two sections. Parshat Zachor discusses the mitzvot regarding Amalek. These are the mitzvot to remember Amalek and to destroy the nation. The reading of Purim does not describe these commandments.
Rav Soloveitchik continues his analysis with a very simple question. What is the nature of this mitzvah to remember Amalek? In his Mishne Torah, Maimonides implies that this commandment to remember Amalek is closely linked to the mitzvah to destroy the nation. Maimonides explains that we are required to destroy Amalek. Then, he adds that we are required to regularly recall the evil of Amalek in order to evoke an abhorrence of this nation. Maimonides seems to imply that remembering Amalek is a precursor to waging war against the nation. We remember Amalek in order to motivate us to fulfill the commandment to destroy Amalek.
This implication is confirmed by Maimonides’ formulation of the mitzvah to destroy Amalek in his Sefer HaMitzvot. There, Maimonides writes that we are obligated to recall the evil of Amalek in order to motivate the Bnai Yisrael to wage war with this wicked nation.
Rav Soloveitchik suggests that Maimonides’ formulation of the mitzvah to remember Amalek suggests that Parshat Zachor may be specifically required. It is possible that the Purim reading is not adequate. The mitzvah to remember Amalek is designed to provide motivation for waging war. It is reasonable to assume that the mitzvah can only be fulfilled through a Torah reading that specifies the obligation to destroy Amalek. Through this reading, the recollection of Amalek’s wickedness is linked to the commandment to destroy the nation. The Purim reading does not discuss the requirement to wage war against Amalek. This commandment is only mentioned in Parshat Zachor.
“One is obligated to read the Megilah at night and to repeat it during the day…” (Shulcah Aruch, Orech Chayyim 687:1)
Shulchan Aruch explains that the Megilah is read twice on Purim. It is read at night and during the day. This law is derived from the Talmud in Tractate Megilah. Tosefot and many other commentaries explain that the two readings of the Megilah are not of equal importance. The more fundamental reading is during the day. There are numerous proofs for this assertion. One simple proof is that the fundamental mitzvot of Purim are observed during the day. For example, the Purim feast can only be held during the day. The Talmud equates these observances to the reading of the Megilah. The equation seems to imply that, just as other mitzvot performed of Purim must be performed during the day, so too the reading of the Megilah is related to the day of Purim and not the night. 
This raises an interesting question. Why, then is the Megilah read at night? Secondly, the wording of Shulcah Aruch and the Talmud seem to imply that the nighttime reading is the more fundamental. Both refer to the daytime reading as a repetition of the nighttime reading. Referring to the second reading as a repetition indicates that it is secondary!
Rav Naftali Tzvi Yehudah Berlin (Netziv) Zt”l answers this question through a brilliant explanation of the relationship between the two readings. In order to understand his explanation, we must more carefully study the text of the Talmud.
The discussion in the Talmud begins by quoting Ribbee Yehoshua ben Levi. He explains that a person is required to read the Megilah at night and lesshnotah by day. The term lesshnotah can be interpreted in two ways. It can mean “to learn” or it can be understood as “to repeat”. At first the Talmud understands the term to mean “to learn”. According to this interpretation, we are required to read the Megilah at night and the study the laws during the day. The Talmud rejects this interpretation and concludes that lesshnotah means “to repeat”. Therefore, the requirement is to read the Megilah at night and repeat the reading during the day.
Netziv asks, “How could the Talmud initially assume that the Megilah is not read during the day?” Yet this seems to be the Talmud’s original understanding of Ribbee Yehoshua ben Levi’s lesson. The Talmud interprets his statement to mean that the Megilah is read at night and the laws of Purim are studied during the day!
Netziv responds that the Talmud never assumed that the laws of Purim should be learned to the exclusion of reading the Megilah. The Talmud always understood that the fundamental reading of the Megilah takes place during the daytime. Instead, the Talmud originally assumed that Ribbee Yehoshua ben Levi was establishing an additional requirement. Beyond the mere reading to the Megilah, one must study the laws. This enriches the reading of the Megilah. Through the study of the laws, the student acquires a more advanced comprehension of the Megilah’s contents. Netziv further points out that this initial interpretation of Ribbee Yehoshua ben Levi’s dictum reveals an essential premise of the Talmud. The Talmud assumes that Ribbee Yehoshua ben Levi is not describing the fundamental mitzvah of reading the Megilah. The fundamental mitzvah is to merely read the Megilah during the day! Ribbee Yehoshua ben Levi is establishing a requirement to enhance this performance.
Through identifying the Talmud’s premise, Netziv answers our questions. The Talmud rejects its initial interpretation of Ribbee Yehoshua ben Levi’s lesson. His intention is to require the reading of the Megilah at night and its repetition during the day. However, the Talmud never abandons its essential premise! Ribbee Yehoshua ben Levi is establishing a requirement to enhance the performance of the mitzvah. In order to enhance the reading during the day, it must be preceded by a reading during the night. The daytime reading will be a repetition of the nighttime reading. Like any material, the Megilah more is understood more clearly with review! Because the daytime reading is a second review, it will be better understood and appreciated.
Netziv explains that the nighttime reading is required to prepare us for the daytime reading. The daytime reading must be a repetition of the nighttime reading. True, the Talmud and Shulchan Aruch refer to the daytime reading as a repetition. However, this is not intended to diminish the importance of this second reading. The intention is to stress its fundamental nature. Through rendering this daytime reading into a repetition it is enhanced with greater understanding and appreciation.
 Rabbaynu Moshe ben Maimon (Rambam / Maimonides) Mishne Torah, Hilchot Melachim 5:5.
 Rabbaynu Moshe ben Maimon (Rambam / Maimonides) Sefer HaMitzvot, Mitzvat Aseh 189.
 Rav Michel Sherkin, Harrai Kedem, Chapter 195.
 Mesechet Megilah 4a.
 Tosefot, Mesechet Megilah 4a.
 Rav Naftali Tzvi Yehuda Berlin (Netziv), Meromai Sadeh, Commentary on Mesechet Megilah 4a.