(a) TOSFOS and the RASHBAM (DH Amar Lo) understand the Gemara in a
straightforward way. They explain that the Gemara concludes in accordance
with Rav Huna's explanation of the Mishnah. Rav Huna stated that Rebbi Yosi
-- who holds that on Erev Pesach one may begin eating through the day until
nightfall -- agrees that on Erev Pesach, one may not begin eating after the
ninth hour. Therefore, the Halachah is that one may not begin eating on Erev
Pesach after the ninth hour, since both Rebbi Yehudah and Rebbi Yosi are in
agreement. Regarding eating on Shabbos after the ninth hour, the Halachah
follows Rebbi Yosi who says that it is permitted, because the Halachah
follows Rebbi Yosi whenever he argues with one of his colleagues, and
because the Halachah follows the opinion of our anonymous Mishnah, which Rav
Huna said is the opinion of Rebbi Yosi (and the Mishnah implies that only on
Erev Pesach must one refrain from eating, but not on Erev Shabbos).
As far as stopping at the onset of nightfall, the Gemara says that regarding
Shabbos we rule like Rebbi Yosi who says that one does not have to stop.
However, Shmuel was stringent and required one to be Pores Mapah and recite
Kidush (but not to actually stop the meal and recite Birkas ha'Mazon).
Regarding stopping at the onset of Pesach, the Halachah follows Rebbi
Yehudah's opinion, that one must stop, as the Gemara said in the name of
Rebbi Yochanan (or Rebbi Yosi bar Chanina).
(b) The ROSH, however, suggests that we may be more lenient with regard to
stopping a meal on Erev Pesach with the onset of Pesach. He asserts that
there is no reason to rule like Rav Huna (who said that Rebbi Yosi agrees to
Rebbi Yehudah's ruling that one may not begin eating after the ninth hour on
Erev Pesach). Although the Gemara discusses Rav Huna's explanation of the
Mishnah at length and seems to reject the explanation of Rav Papa (who said
that the Mishnah is expressing the opinion of Rebbi Yehudah, and *not* Rebbi
Yosi, and Rebbi Yehudah holds that on Erev Pesach, one may not eat from the
ninth hour, while on Erev Shabbos one may not eat from nine and a half
hours), nevertheless there is significant grounds for following Rav Papa's
The only reason the Gemara rejected Rav Papa's explanation was because of a
Beraisa in which Rebbi Yehudah states that it is forbidden to eat on Erev
Shabbos from the *ninth* hour. Rav Papa had said that Rebbi Yehudah only
prohibits eating on Erev Shabbos from nine and a half hours, and therefore
the Gemara rejects his answer. However, while that Beraisa contradicts Rav
Papa, there is another statement that supports Rav Papa -- "Halachah k'Rebbi
Yehudah b'Erev Pesach." The simple understanding of that statement implies
that Rebbi Yosi argues with Rebbi Yehudah *even* with regard to eating on
Erev Pesach. The Gemara, in attempting to answer Rav Huna, had to say that
that statement refers to *stopping* on Erev Pesach. That was a forced
answer, though, and it is certainly not the simple understanding of the
statement. According to Rav Papa, the statement makes sense by itself,
providing strong grounds to rule like Rav Papa. Even though a Beraisa
contradicts Rav Papa, the Beraisa may have been recorded in error (as the
Gemara itself suggested at the top of this Amud). If the Halachah is like
Rav Papa, Rebbi Yosi argues with Rebbi Yehudah even with regard to starting
a meal on Erev Pesach.
Logically, too, there is more reason to differentiate between *beginning* to
eat on Erev Pesach and beginning to eat on Erev Shabbos, than to
differentiate between *stopping* one's meal on Erev Pesach and stopping on
Erev Shabbos, the Rosh asserts. Regarding beginning to eat, on Erev Pesach
there is a concern that one must be able to eat Matzah on Pesach night with
an appetite. There is no such concern on Erev Shabbos. Regarding stopping at
nightfall, though, Building an appetite plays no role. Rather, the question
is one of honoring the holiday by stopping one's meal to make Kidush. Both
Erev Pesach and Erev Shabbos are the same in that respect (or, if anything,
the honor of the Shabbos is more important than Pesach). This is the
reasoning of the Rosh, who concludes that Rav Papa's is the Halachic ruling.
Therefore, on Erev Shabbos, the Halachah follows Rebbi Yosi -- just as
Tosfos and the Rashbam hold -- and one is permitted to begin eating all day,
and when Shabbos arrives one does not need to end his meal. On Erev Pesach,
too, the Rosh agrees that one is not allowed to begin eating after the ninth
hour, not because that is the opinion of Rebbi Yosi (as Tosfos and the
Rashbam write), but because that is the opinion of Rebbi *Yehudah*, and the
Gemara says that we follow Rebbi Yehudah with regard to *beginning a meal*
on Erev Pesach.
However, when it comes to stopping one's meal, we rule like Rebbi Yosi with
regard to both Erev Shabbos *and Erev Pesach* (and it suffices to be Pores
Mapah and recite Kidush without beginning a new meal), because the Rosh
learns that when the Gemara says we rule like Rebbi Yehudah with regard to
Erev Pesach, it is talking only about beginning a meal, but not stopping.
(c) TOSFOS (DH Ein Mafsikin) quotes the BEHAG who modifies Tosfos' ruling
l'Chumra, with regard to beginning a meal on Erev Shabbos. Behag rules that
on Erev Shabbos, it is prohibited to eat from nine hours, *even according to
Rebbi Yosi*. He holds that Rebbi Yosi only permits eating then *b'Di'eved*
(i.e. if one started eating after nine hours, he does not have to stop), but
l'Chatchilah he agrees that one may not start.
How can the BEHAG say that? It is clear in the Gemara (100b) that Rebbi Yosi
permits one even to *begin* eating on Erev Shabbos after nine hours (see
Rashbam there, DH Kan l'Achar Tesha)!
The VILNA GA'ON (OC 249:7 and 529:4) explains that since Rebbi Yosi states
that "one does not need to stop his meal *if he already started to eat*," it
must be that he holds that one should not start eating l'Chatchilah. What
about the Gemara that says that Rebbi Yosi holds that one may start eating
l'Chatchilah? The Vilna Ga'on explains that there are two different meals
being discussed. When the Behag says that Rebbi Yosi holds that one may not
begin eating, he means that it is not permitted to begin *the type of meal
which one normally eats during the week*. The Gemara that says that one may
begin eating is referring to a *small meal*!
This is indeed the way the RAMBAM rules in Hilchos Shabbos (30:4) and in
Hilchos Yom Tov (6:16).
It should be noted that included in this prohibition is beginning to eat
before the ninth hour *with intent* to continue until after the ninth hour
(RASHBAM 107b, DH Sof Sof). If one intends to continue until after the time
at which it becomes prohibited to eat, it is equivalent to starting the meal
at that time.
(This may apply only if the only if the meal was begun b'Heter, after the ninth hour,
see Chart #17.)