When during the Friday night prayer service do the laws of Shabbat take effect? At one specific point during the service does it become forbidden for a person to perform Melacha (forbidden activity)?
This issue is subject to a dispute between Maran (Rav Yosef Karo, author of the Shulhan Aruch) and the Arizal (Rav Yishak Luria of Safed, 1534-1572). The Shulhan Aruch maintained that one formally accepts Shabbat when he recites the Psalm of "Mizmor Shir Le炭om Ha担habbat," whereas the Arizal held that one accepts Shabbat earlier, when he recites "Bo段 Kalla Shabbat Malketa." The Ben Ish Hai (Rav Yosef Haim of Baghdad) accepted the stringent view of the Arizal, and Hacham Ovadia Yosef, in his work Halichot Olam, writes that one should preferably follow this view. Therefore, one should not perform any Melacha after reciting the words "Bo段 Kalla Shabbat Malketa" at the end of Lecha Dodi (even though the sun has not set).
It should be noted that responding to Barechu at Arbit on Friday night also constitutes the formal acceptance of Shabbat. Of course, in most cases, one has already accepted Shabbat before Barechu by reciting "Bo段 Kalla Shabbat Malketa," as discussed above. However, if it happens that one responds to Barechu of Arbit before reciting "Bo段 Kalla Shabbat Malketa," he has nevertheless accepted Shabbat. This can happen in places such as the Kotel Ha知a誕rabi (Western Wall) in Jerusalem, where many different Minyanim are taking place at the same time. If a person hears a Minyan praying Arbit on Friday night, and he responds to Barechu, he has accepted the onset of Shabbat, regardless of whether or not he has recited Kabbalat Shabbat. In fact, if a person responds to Barechu on Friday night before he recited Minha, he may no longer recite Minha. Since he has accepted the onset of Shabbat through his response to Barechu, he cannot then recite the weekday Minha prayer. This applies even though he responds to Barechu with the specific intention not to accept Shabbat. Therefore, if a person has yet to recite Minha on Friday afternoon and hears Barechu from a Minyan praying Arbit, he should not respond, because he would then be unable to pray Minha.
This Halacha is restricted to Barechu on Friday night. During the week, responding to Barechu does not mean that one has formally ended the day and begun the night. Therefore, if a person has yet to recite Minha and hears a Minyan praying Arbit, he may respond to Barechu and then pray Minha. Conversely, a person who recited Arbit and then hears a Minyan praying Minha may respond to Nakdishach, even though he has already recited Arbit. Reciting Arbit, or answering to Barechu of Arbit, does not constitute the "acceptance" of the nighttime period, and therefore one may still respond to Nakdishach after reciting Arbit, or pray Minha after responding to Barechu of Arbit.
Summary: Once a person recites "Bo段 Kalla Shabbat Malketa" at the end of the Lecha Dodi hymn, he is considered to have accepted Shabbat and may no longer perform activity forbidden on Shabbat, even though the sun has not set. A person who answers to Barechu of Arbit on Friday night is considered to have accepted Shabbat even though he has not recited Kabbalat Shabbat. If a person hears a Minyan praying Arbit on Friday night before he has prayed Minha, he should not respond to Barechu, as he would then be considered to have accepted Shabbat and would thus be unable to pray Minha.
