1) The time it takes to say the words ''Shalom Alecha Rebbi'' (Mishna Berurah 487:4), or according to the Pri Megadim ''Shalom Alecha Rebbi U'Moree.''

1) Exodus 20:8. 2) The Hebrew word ''Zachor'', which is usually translated as ''remember'', can also mean ''talk about'' in certain contexts. 3) In other words, both Kiddush and Havdala are aspects of one Mitzvah, derived from a single verse: ''Remember the Shabbat day to sanctify it.''.

מצות עשה מן התורה לקדש את יום השבת בדברים שנאמר זכור את יום השבת לקדשו כלומר זכרהו זכירות שבת בקידוש וצריך לזכרהו בכניסתו בקידוש וגם ביציאתו בהבדלה ותקנו חכמים שתהא זכירה זו על כוס יין בין בכניסתו בין ביציאתו

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 (the size of an olive) after that point in time; these authorities hold that the mitzvah to eat a meal on Shabbat has to be fulfilled on Shabbat itself, in other words, after nightfall. The Mishna Berurah (267:5) writes that one should try to follow their ruling. 2) In Chapter 76 Law 1, there is a dispute in the Talmud as to what is the correct time frame within which to say the afternoon prayers and the evening prayers. Rabbi Yehuda rules that one can only say the afternoon prayers up to "Plag HaMincha," which is one and a quarter seasonal hours (Shaot Zemaniot) 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 say the evening prayers. The Rabbis, however, rule that the time for the afternoon prayers 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 say the evening prayers 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 Shabbat early. Firstly, most authorities rule that it is a Mitzvah to begin Shabbat while it is still considered to be ''daytime'' on Friday, i.e., before sunset. This concept is called ''Mosif Mechol Al HaKodesh'' (adding part of the non-holy to the holy). Secondly, even those authorities who rule that it is not an actual Mitzvah to begin Shabbat 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 Shabbat, 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 say the evening Shema until after the appearance of three stars. Consequently, according to these authorities, since one prayed the Shabbat evening prayers before the appearance of the stars, one did not fulfill one's obligation on the Shema by reading it during the evening prayers, and therefore must delay the meal until after the appearance of the stars, when one will be able to say the Shema in its proper time. 5) The Mishna Berurah (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 Shabbat commences, even when one accepts Shabbat before nightfall. There are authorities who rule that one may be lenient if one's thirst is causing discomfort.