Many communities have the practice of accepting Shabbat and reciting Arbit before sundown on Friday afternoon during the summer months. Rather than waiting until after dark to recite Arbit, as we normally do, these communities recite Arbit and begin Shabbat after Pelag Ha'minha (approximately one and a quarter hours before sundown).
If one begins Shabbat before sundown, may he recite Kiddush and begin his Shabbat meal immediately, or must he wait until dark?
The Shulhan Aruch (267:2) rules explicitly that even when one begins Shabbat before sundown, he may nevertheless recite Kiddush and conduct his meal immediately, and need not wait until nightfall. However, the Mishna Berura (267:5) cites some authorities who require that one eat at least a Kezayit (approximately 1 oz.) of bread after nightfall. According to this view, the obligation to eat three meals on Shabbat requires that they be eaten on Shabbat itself, and not during the period on Friday afternoon that one added onto Shabbat. Hence, one should ensure to eat at least a Kezayit of bread the minimum amount that constitutes a "meal" after dark, in order to fulfill the obligation of the Shabbat meal. (Hacham Ovadia Yosef cites this stringent position, as well.)
Often, when a person accepts Shabbat early, he returns home and begins his meal within a half-hour of nightfall, in which case an interesting Halachic question arises regarding the obligation of Keri'at Shema. The Mitzva to recite the evening Shema begins at Tzet Ha'kochavim (nightfall), and Halacha forbids beginning a meal within a half-hour of Tzet Ha'kochavim out of concern that one might forget to recite Shema as a result. Thus, if a person returns from the synagogue on Friday night within a half-hour of nightfall, he should perhaps be required to wait until after nightfall, recite Shema, and only then begin his meal.
Maran (author of the Shulhan Aruch) addresses this question in his work Bet Yosef, where he rules that one may, in fact, begin his Shabbat meal within a half-hour of nightfall on Friday night. According to the Rosh (Rabbenu Asher Ben Yehiel, Germany-Spain, 1250-1327), one may fulfill the evening Shema obligation before sundown, so long as he recites the Shema after Pelag Ha'minha. Although Halacha does not follow this view, and requires that the evening Shema be recited specifically after nightfall, we may nevertheless rely on this view with respect to the prohibition against beginning a meal within a half-hour of nightfall. Once a person has recited Arbit and has thus fulfilled his Shema obligation according to the Rosh's view, he may then begin his meal within a half-hour of Tzet Ha'kochavim. (Needless to say, when one recites Arbit before nightfall he must repeat the Shema after nightfall.)
Summary: When one accepts Shabbat and recites Arbit on Friday afternoon before sundown, as is customary during the summer months, he may recite Kiddush and begin the Shabbat meal immediately upon returning from the synagogue. He must repeat the Shema after nightfall, and, according to some authorities, he should eat at least a Kezayit (1 oz.) of bread after nightfall.
