QUESTION: The Mishnah states that on Erev Pesach, one may not eat from "close to Minchah... until dark." The Gemara cites a similar Beraisa in which Rebbi Yehudah says that this applies both to Erev Shabbos and to Erev Yom Tov. The Beraisa, though, leaves out the words "until dark." Is there a difference between the requirement to refrain from eating on Erev Pesach and the Halachah to refrain from eating on Erev Shabbos?
(a) TOSFOS writes that on Pesach there is a special requirement to eat Matzah at the time of the Korban Pesach, after nightfall. Therefore, the Mishnah says that one may not eat "until dark." On Shabbos, however, a person may accept the sanctity of Shabbos, recite Kidush, and begin his meal even before nightfall. The ROSH agrees with this opinion.
(b) The BACH (beginning of OC 472) cites the MAHARAL (Gevuros Hash-m 48) who writes that while one may begin his Shabbos meal before nightfall, he must eat part of his meal after dark. The source of the Mitzvah to eat three meals on Shabbos is the verse which mentions the word "ha'Yom" ("the day") three times (Shemos 16:25; see Shabbos 117b). The time before nightfall is not yet called the day of Shabbos. Therefore, at least part of the meal must be eaten on Shabbos after nightfall.
(c) TOSFOS (DH Ad) cites RABEINU YEHUDAH who quotes the Yerushalmi that Erev Shabbos and Erev Pesach are the same; just as one may not eat until after dark on Erev Pesach, so, too, one may not eat on Erev Shabbos until after dark. Therefore, the Beraisa which discusses Erev Shabbos should also say "Ad she'Techshach" ("until dark"). Apparently, one may not eat on Erev Shabbos until after dark because one cannot fulfill the Mitzvah to declare the sanctity of Shabbos ("Zachor Es Yom ha'Shabbos") when the day of Shabbos has not yet arrived.
HALACHAH: On Erev Pesach, one must wait until dark to recite Kidush at the beginning of the Seder (SHULCHAN ARUCH OC 472). The TERUMAS HA'DESHEN (#137) reasons that the four cups of wine, which include the cup of Kidush, represent the Ge'ulah which is celebrated Pesach night. Since the Ge'ulah occurred after nightfall, the Seder must wait until that time as well.
With regard to Shabbos, the SHULCHAN ARUCH (OC 267:2) writes that one may begin his Shabbos meal before dark, as long as he starts after Plag ha'Minchah (1 1/4 hours before sunset). The MISHNAH BERURAH quotes the view of the BACH in the name of the MAHARAL (in (b) above), that if one eats his Shabbos meal early, he should eat at least a k'Zayis of bread after nightfall.
The Mishnah Berurah raises another issue. The MAGEN AVRAHAM (OC 233) writes that one is not permitted, on any night, to begin a meal within a half-hour before nightfall because he might get involved in his meal and forget to recite Keri'as Shema after dark. Therefore, if one does recite Kidush and begin the Shabbos meal early, he must start prior to a half-hour before nightfall (but, again, after Plag ha'Minchah). The Mishnah Berurah cites the TAZ (OC 233 and 271) who, on the contrary, permits a person to start his meal even within a half-hour of nightfall, as long as he has already recited Keri'as Shema. This is based on the opinion noted by TOSFOS (Berachos 2a) that one may say Keri'as Shema before nightfall (see Insights to Berachos 2:2).
However, the Mishnah Berurah (in BI'UR HALACHAH 271:1 and SHA'AR HA'TZIYUN 267:1) points out that the ruling of the Taz applies only to people who have a Minhag to always recite Ma'ariv early. One who always recites Ma'ariv after sunset is not permitted to recite Keri'as Shema early on Friday evening and begin the Shabbos meal early.
(This discussion applies only to eating the Shabbos meal before dark. With regard to accepting upon oneself the sanctity of Shabbos before dark, the SHULCHAN ARUCH (OC 261:2) cites the opinion that it is a Mitzvah to add to the Shabbos from the weekday ("Tosefes Shabbos"). See Mishnah Berurah there (261:19).)