The Gemara in Masechet Pesahim relates that Raba had the practice of drinking wine on Ereb Pesah, because wine has the effect of arousing an appetite. The Masa and other foods at the Seder should be eaten with a hearty appetite, and Raba therefore made a point of drinking wine during the day of Ereb Pesah in order to increase his appetite.
On the basis of this Talmudic passage, the Tur (Rabbenu Yaakob Ben Asher, Germany-Spain, 1269-1343) ruled in Siman 471 that it is permissible to drink wine in any amount on Ereb Pesah. This also appears to be the view of the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), who does not specify any particular amounts of wine that are permissible on Ereb Pesah. This was the view of the Rosh (Rabbenu Asher Ben Yehiel, Germany-Spain, 1250-1327), as well, as cited by the Be’ur Halacha (supplemental essays to the Mishna Berura).
Tosafot (Medieval French and German Halachic scholars), however, disagree, and maintain that only large quantities of wine have the effect of generating an appetite. Drinking wine is small amounts, Tosafot claim, have the opposite effect, of bringing a feeling of satiation, and they thus rule that one may only drink wine in significant quantities on Ereb Pesah, and not in small quantities. This is the ruling codified by the Shulhan Aruch (Orah Haim 471; listen to audio recording for precise citation).
How do we define "large" and "small" quantities with respect to this Halacha?
The Mishna Berura (commentary by Rav Yisrael Meir Kagan of Radin, 1839-1933) cites the later authorities who explain that a "small quantity" is the range between the majority of a Rebi’it and the majority of two "cups." A Rebi’it is generally assumed to be 3.2 oz, and thus the majority of a Rebi’it would be approximately 1.7 oz. Two "cups" measure 6.4 oz, and thus the majority of this amount is approximately 3.2 oz. Hence, it would be forbidden on Ereb Pesah to drink between 1.7 and 3.2 oz of wine. Drinking less than this quantity does not have any effect, and exceeding this amount will actually enhance one’s appetite, and is thus permissible. It is only amounts ranging from 1.7 oz and 3.2 oz that would be forbidden as it may likely reduce one’s appetite.
Of course, one should certainly not drink wine excessively on Ereb Pesah to the point where he is intoxicated; quite obviously, one should not go into the Seder in a state of inebriation.
The Nit’eh Gabriel (Rav Gavriel Zinner, contemporary) cites authorities who maintain that grape juice has the same status of wine in this regard. Thus, if one wishes to drink grape juice on Ereb Pesah, he should ensure to drink either less than 1.7 oz or more than 3.2 oz.
Other drinks – besides wine and grape juice – are permissible in any quantity on Ereb Pesah.
It should be noted that this entire discussion refers only to the late afternoon hours of Ereb Pesah, from the end of the ninth hour and on. Generally, this period begins around 3-4 pm, depending on one’s location. It is permissible to drink wine or grape juice in any quantity earlier than this time on Ereb Pesah.
Furthermore, the Mishna Berura (listen to audio recording for precise citation) notes that this Halacha also depends upon one’s nature. If one knows that his appetite would be adversely affected by drinking wine, then he should certainly refrain from doing so. Since this entire Halacha is geared toward ensuring a hearty appetite at the Seder, one must take into account his personal tendencies and exercise common sense when determining what and how much he eats and drinks in the afternoon of Ereb Pesah.
Finally, the Shulhan Aruch rules that it is forbidden to eat "Pat" starting from the end of the ninth hour of Ereb Pesah. The term "Pat" generally means "bread," but this certainly cannot be the meaning in this context, since bread is forbidden already from the midmorning hours on Ereb Pesah. It cannot refer to Masa, either, because Masa that is suitable for the Misva at the Seder is forbidden throughout the day of Ereb Pesah, already from the morning. Therefore, "Pat" here must refer to "Masa Ashira," or egg Masa. Even though Sephardic custom allows eating egg Masa on Ereb Pesah, it should not be eaten after the end of the ninth hour, in order to ensure that one has an appetite at the Seder. One should also refrain from "Mezonot" foods after the end of the ninth hour, for the same reason. One may, however, eat fruits, vegetables and meat after this point, but even so, as the Rambam writes, one should not eat large amounts to the point of satiation.
Summary: Starting from the end of the ninth hour of Ereb Pesah, which occurs around 3-4 pm, one may not eat egg Masa or "Mezonot" food. During this period, one may also not drink wine or grape juice, unless he drinks less than 1.7 oz or more than 3.2 oz. More generally, one should ensure to have an appetite for the Seder, and thus should avoid eating or drinking to an extent that will affect his appetite.
