The Shulchan Aruch (695:1) writes: “If one makes his Purim seudah by night he doesn’t fulfill his duty.” The Rema adds: “Nonetheless, at night one should increase in simcha a little bit and have a small seudah”. The Mishnah Berurah (695:3) writes: “Even when the 14th of Adar falls on motzei Shabbos, and one has had a chosuvah seudah for seudas shlishis, one should still increase slightly in honor of Purim”.
The Machatzis HaShekel (695:1) writes a big chiddush:דהחכם עיניו בראשו למעט קצת באכילתו ליתן מקום לסעודת ליל פורים – “One who has foresight will reduce what he eats (for seudas shlishis) in order to have space for the meal on Purim night”.
We find a similar thing in the Rema (529:1) who writes: “When erev Yom Tov falls on Shabbos, one should eat seudas shlishis, however, he should only eat a little bit of bread in honor of Yom Tov”. The Mishnah Berurah explains (s.k. 8) the reason one should only eat a little bit, is so that he has appetite at night. The Magen Avraham adds: When first day Yom Tov falls on Shabbos, one should do the same thing, so that he has an appetite for the night meal of second day Yom Tov. The Machatzis HaShekel understands that when Purim falls on motzei Shabbos it’s the same thing.
However, the Shu”t Betzel HaChochma (3:5) points out, there is a big difference between the two cases. When it comes to Yom Tov Sheini, although we now can work out the months and know exactly when Yom Tov falls, there is a chiyuv derabonon to make kiddush and eat a Yom Tov seudah on second night Yom Tov. Eating a meal on Purim night, however, even according to the Bach (mentioned in Mishnah Berurah 695:3) who holds, that even if one had a chosuvah seudah for seudas shlishis he should still have a small seudah on Purim night, there is no a strict chiyuv, so where does the Machatzis HaShekel get it from that one should reduce in seudas shlishis to ensure one has space for a meal on motzei Shabbos (Purim night)?
The Betzel HaChochma asks further: The Biur Halachah (529, d.h. B’erev Yom Tov) asks on the Magen Avraham who says one should reduce what he eats on first day Yom Tov to save room second day Yom Tov. He asks, that first day Yom Tov is the main day of Yom Tov, so why should one reduce his enjoyment on first day Yom Tov, in order that he has enjoyment on the second day? If the Biur Halachah struggles to understand reducing what one eats on first day Yom Tov, in order one has appetite on second day Yom Tov, then certainly, one shouldn’t need to reduce what he eats for seudas shlishis, in order that he has appetite for a meal on Purim night. Especially, when the mitzvah of oneg Shabbos is de’O’raisa, or at least divrei kabbolah and the meal on Purim night is just a nice thing.
Therefore, the Betzel HaChochma explains, the Machatzis HaShekel is simply giving some useful advice, teaching you a simple way to ensure that one is able to fulfill both mitzvos. If, however, one is actually hungry on Shabbos, certainly it’s ok to eat a lot for seudas shlishis, as oneg Shabbos, is de’O’raisa, or at least divrei kabbolah, whereas the seudah on Purim night is nothing more than a slight mitzvah and nice thing to do.
In Minhagay Chasam Sofer (cited in the introduction to the Sifsei Chachomim on Megillah, Perek 9, ois 21) it’s brought down, that when erev Purim fell on Shabbos, the Chasam Sofer would daven Mincha early Shabbos afternoon straight after his meal, and he would then have seudas shlishis before the tenth hour of the day. He was also accustomed to doing this when Yom Tov fell on Sunday. It would seem that he was in fact worried about what we mentioned above.
