QUESTION: Rebbi Ami and Rebbi Yirmeyah disagree about how each of the Four Parshiyos are read on Shabbos. Rebbi Ami maintains that the normal Parshah of that week is postponed and only the appropriate Parshah from the Four Parshiyos is read. Rebbi Yirmeyah maintains that the normal Parshah of that week is read, and the Parshah from the Four Parshiyos is read as the Maftir by the last Aliyah, who also reads a Haftarah which deals with the same subject.
Abaye adduces proof for the opinion of Rebbi Ami from the wording of the Mishnah (29a). The Gemara refutes his proof.
It is evident from the Gemara here that Abaye sides with the view of Rebbi Ami. However, in the Gemara earlier (30a), Abaye expresses his opinion about how to read Parshas Shekalim when it coincides with the normal weekly reading of Parshas Tetzaveh or Parshas Ki Sisa. Abaye says that the first six Aliyos read the normal Parshah of that week, and the seventh Aliyah reads from Parshas Shekalim. If Abaye rules, like Rebbi Ami, that the only portion read on each Shabbos of the Four Parshiyos is the special Parshah of the Four Parshiyos, then why does he rule that the first six Aliyos read from the normal Parshah of that week?
ANSWER: The VILNA GA'ON (in BI'UR HA'GRA OC 685:2) explains that Abaye must have retracted his original opinion and sided with Rebbi Yirmeyah and not with Rebbi Ami. His final ruling was that six Aliyos read from the normal Parshah of the week and the last Aliyah reads from the Arba Parshiyos. The Poskim rule in accordance with the view of Rebbi Yirmeyah, because both Rebbi Yitzchak Nafcha and Abaye in the Gemara earlier agree with him.
QUESTION: The Mishnah lists the Torah readings for the festivals. On Pesach, Rosh Hashanah, and the first day of Sukos, we read the section of Parshas Emor (Vayikra 23) which discusses those Mo'adim. On Shavuos, we read the section in Parshas Re'eh (Devarim 16:9) which discusses Shavuos.
Why do we not read on Shavuos the section in Parshas Emor which discusses Shavuos, just as we read from Emor on Pesach, Rosh Hashanah, and Sukos?
ANSWER: The RAN answers that we do not read from Emor on all of the festivals because we want to alternate the readings from the two Parshiyos which discuss the Mo'adim. On Pesach we read from Parshas Emor, and on the next festival, Shavuos, we read from Parshas Re'eh. Then, on Sukos, we read from Parshas Emor again. (The following festival, Pesach, begins a new year of Mo'adim, and thus we begin again from Parshas Emor.)
The Ran's rotation system, however, does not seem to take into account the reading of Rosh Hashanah. On Rosh Hashanah, we read from Emor, and thus on Sukos we should read from Parshas Re'eh. Why do we read again from Parshas Emor on Sukos?
The answer to this question may be found in the MORDECHAI. The Mordechai infers from the wording of the Mishnah that on Rosh Hashanah we do not read from Parshas Emor the entire section of the Mo'adim, as we do on Pesach and Sukos. Rather, we read only the verses which relate specifically to Rosh Hashanah. When the Mishnah refers to the reading of Rosh Hashanah it says that we read the verses of "ba'Chodesh ha'Shevi'i" (in Parshas Emor), but when it refers to the reading of Pesach and Sukos it says that we read "Parshas Mo'ados" in Parshas Emor. (Although the Mishnah says with regard to Shavuos that we read "Shiv'ah Shavuos" and not "Parshas Mo'ados," it is clear from the current practice (on the second day of Shavuos outside of Eretz Yisrael, see 31a) that the Mishnah means that we read the entire section of the Mo'ados on Shavuos and not just the part which discusses Shavuos. The inference of the Mordechai regarding Rosh Hashanah is based on the change in the wording of the Mishnah. The Mishnah would not refer to the same Parshah -- Parshas Mo'ados of Emor -- by two different names.)
Since the Rosh Hashanah reading comprises only a few verses of the section in Emor which discusses the festivals, the Torah reading of Rosh Hashanah does not count as part of the rotation of Emor-Re'eh. Therefore, on Sukos we read the full section of the Mo'adim in Parshas Emor.
(Why indeed is the entire Parshas Mo'ados read on Pesach and Sukos but not on Rosh Hashanah? Perhaps the reason is because only sections that are relevant to the festival may be read on that festival. The three Regalim -- Pesach, Shavuos, and Sukos -- are related to each other in that they share the common obligations of Aliyah l'Regel and Korban Re'iyah. Therefore, the Torah reading on each of those festivals may include mention of the other two festivals. Rosh Hashanah, in contrast, is not directly related to the three Regalim, and therefore it is not appropriate to read the entire Parshas Mo'ados on Rosh Hashanah.)