Kiddush and the Shabbos Meals
2. It is permissible to recite Kiddush and eat the Shabbos meal even before nightfall (1). However, those individuals who generally recite Maariv at its proper time (after the appearance of the stars) (2), but recite it earlier on Shabbos (3), are forbidden to eat from half an hour before the appearance of the stars onwards (4). Accordingly, if they have not [begun their Shabbos meal] before this time arrives, they must wait until the appearance of three stars, at which time they should recite the three portions of the Shema, and only then recite Kiddush and begin the meal (5).
It is forbidden to eat or drink anything, even water, before reciting Kiddush (6).
(1) There are those who hold (Sefer Chassidim, Bach) that, although one may begin the meal before nightfall, one should make sure that the meal continues until after nightfall, and that one should eat some bread (size of an olive) after that point in time; these authorities hold that the mitzvah to eat a meal on Shabbos has to be fulfilled on Shabbos itself, in other words, after nightfall. The Mishna Berura (267:5) writes that one should try to follow their ruling.
(2) As we saw in Halacha -Yomi 76:1, there is a dispute in the Talmud as to what is the correct time frame within which to recite Mincha and Maariv. Rabbi Yehuda rules that one can only recite Mincha up to "Plag HaMincha," which is one and a quarter seasonal hours ("Shaos Zemanios") before sunset (a "seasonal hour" is equal to one twelfth of the total time from sunrise to sunset (some say from dawn to dark) ); from Plag HaMincha onwards, one can recite Maariv. The Rabbis, however, rule that the time for Mincha extends until nightfall (in practical terms, this means sunset, because after that point we are not sure when "night" actually begins), and one can only recite Maariv after nightfall (that is, from the time we are certain that nighttime has begun, which is when 3 stars appear).
(3) There are two reasons for beginning Shabbos early. Firstly, most authorities rule that it is a Mitzvah to begin Shabbos while it is still considered to be "daytime" on Friday, i.e., before sunset. This concept is called "Mosif Mechol Al HaKodesh" (lit: "adding part of the non-holy to the holy"). Secondly, even those authorities who rule that it is not an actual Mitzvah to begin Shabbos early, still rule that everyone should refrain from performing prohibited acts a short time before sunset, because if one got into the habit of waiting until a moment before sunset to begin Shabbos, one could easily end up performing a prohibited act after sunset (See Shulchan Aruch 261 and commentaries).
(4) It is forbidden to begin a meal during the half-hour period leading up to the time for reciting the Shema; most authorities rule that one does not fulfill one's obligation to recite the Evening Shema until after the appearance of three stars. Consequently, according to these authorities, since one prayed the Shabbos Maariv before the appearance of the stars, one did not fulfill one's obligation to recite the Shema during the Maariv service, and therefore must delay the meal until after the appearance of the stars, when one will be able to recite the Shema in its proper time.
(5) The Mishna Berura (267:6) writes that one should not rebuke those who do not delay the meal until after the appearance of stars, because they do have authorities upon whom to rely.
(6) This prohibition begins as soon as Shabbos commences, even when one accepts Shabbos before nightfall. There are authorities who rule that one may be lenient if one's thirst is causing discomfort.