On Purim we read Megillas Esther twice, once by night and once by day. It is uncertain what the nature of the obligation is. Did the rabbanan obligate us to read the megillah as a part of Kesuvim, similar to the obligation of reading the other megillos (such as Eichah and Shir HaShirim) and similar to the reading of the Torah? Or is the obligation to read only for the purpose of publicizing the miracle (pirsumei nisa)?
It says in Maseches Sofrim 14:3 that prior to reading Megillos Rus, Eichah, Shir HaShirim and Esther one must recite the berachah of “…al mikra megillah.” The fact that the Mesechta Sofrim combined all of the megillos into one halacha implies that the obligation to read each of them is the same – namely to read Kesuvim.
The Yerushalmi in Megillah 3:4 says that the reason we are not allowed to read Megillas Esther on Shabbos (when Purim falls out on Shabbos) is because it is forbidden to read Kesuvim on Shabbos. If the obligation to read Megillas Esther were merely for the mitzvah of publicizing the miracle, it would not be considered as if we are reading Kesuvim – and would therefore be permitted on Shabbos. The fact that the Yerushalmi prohibits the reading of Megillas Esther on Purim, when it falls out on Shabbos, clearly indicates that the obligation is to read Kesuvim.
Based on this, the Sefer Harirai Kedem suggests that we can answer the following question: The Gemara in Shabbos 23a and Sukkah 46a ask how we can say “vetzivanu – and He [Hashem] commanded us” in the berachah that we recite on lighting the menorah on Chanukah, for it is only a mitzvah mi’derabbanan. The Gemara answers that if one does not adhere to the command of the rabbanan, it is a transgression of the pasuk in the Torah of “lo sasur.” Therefore one can say that the Torah commanded him to perform this mitzvah. The question is raised: Why does the Gemara not have the same discussion regarding the mitzvah of reading Megillas Esther, which is also a mitzvah mi’derabbanan – but at which time vetzivanu is recited?
If we understand that the berachah of “…vetzivanu al mikra megillah” is a berachah that one recites when reading any megillah that is part of Kesuvim and not for the mitzvah, we can understand why the Gemara does not inquire as to how we can say vetzivanu prior to reading Megillas Esther. Since the berachah is recited even when there is no obligation to read it, the Gemara understood that the berachah is not recited because the rabbanan commanded us to read it, but rather because reading a megillah requires that this berachah be recited.
The Brisker Rav asked the following question: The Shulchan Aruch (Orach Chaim 690:3) rules, based on the Gemara in Megillah 18b, that one must read the entire megillah while looking inside the megillah. If the sofer skipped several words (up to half the megillah) the reader may recite those words by heart. (The Ramah says that this only applies if an entire inyan (topic) was not skipped. Reb Moshe Soloveitchik said that today we are not certain what an entire inyan is, and therefore we cannot rely on this halacha.) Reading Kesuvim must be read while directly looking at the words – just like krias HaTorah. How then can the obligation to read the megillah be to read Kesuvim, if one can read the megillah by heart? Rather, from this halacha, it seems that the obligation is to publicize the miracle.
The Sefer Harirai Kedem explains that the reason one may read part of the megillah by heart is because we apply the rule of rubo kekulo – the majority is considered as if it is the whole megillah. Since the majority of the megillah is written, when one reads the remaining part by heart it is considered as if he read the entire megillah directly from the megillah. The reason we do not apply this rule to krias HaTorah is because the rule can only be applied when the subject matter is a complete item but lacking a part of it. However, if the matter of discussion is not a complete item, even when it is in its entirety, we cannot apply the rule. As Megillas Esther is a complete item, we can apply the rule. But when one must read a certain amount of p’sukim in the Torah, those p’sukim do not combine to create a complete entity on their own. Rather they are only a part of the complete Torah, and therefore the rule is not applied.
If, though, there was a halacha to read the entire Torah at once, we would apply the rule and it would suffice to only read the majority directly from the Torah. Similarly we do not apply the rule when one must eat a specific amount of matzah (a k’zayis) and say that it suffices to eat the majority of the matzah since a k’zayis is not a complete item but rather an amount. As a result, we can suggest that the obligation to read the megillah is to read Kesuvim.