==When does [[Shabbat]] start?==
# It is absolutely forbidden to do any Melacha (activities forbidden on [[Shabbat]]) after sunset on Friday. (See footnote for background)<ref>
* S"A 261:2 writes that one should accept [[shabbat]] early during the time between [[Shekiyah]] (sunset) and [[
Ben HaShemashot]] (twilight) to fulfill the mitzvah of Tofeset [[Shabbat]]. S"A explains that this time period is the length of time it takes to walk 3.25 [[mil]]. Then [[ Ben HaShemashot]] (twilight) lasts for a period of 3/4 of a [[mil]] which is followed by [[Tzet HaKochavim]] (emergence of the stars). For this discussion, we're assuming that a [[mil]] is considered 18 minutes like the ruling of S"A 459:2 and Mishna Brurah 459:15. If so, S"A holds that one must accept [[Shabbat]] 13.5 minutes before [[Tzet HaKochavim]] which is 58.5 minutes after [[Shekiyah]]. * However, Mishna Brurah 261:23 quotes many Rishonim and the Gra who hold that between Sheiyah and [[Tzet HaKochavim]] there is a short [[ Ben HaShemashot]] of 3/4 of a [[mil]] (13.5 minutes) and afterwards it's considered nighttime Deoraitta. According this opinion the Mishna Brurah writes that one may absolutely stop doing Melacha (activities forbidden on [[Shabbat]]) before [[Shekiyah]].
* The 39 Melachos (Rabbi Ribiat, vol 1, pg 145) writes that the accepted practice is in accordance with the Gra and so it's strictly forbidden from doing Melacha from the time of [[Shekiyah]]. Yalkut Yosef ([[Shabbat]], vol 1, pg 128) agrees. </ref>
* However, the Bet Yosef 267:2 argues based on the Rambam ([[Tefillah]] 3:7) and Rosh ([[Brachot]] 4:6) who simply codify the halacha of praying [[Arvit]] early on Friday that one is permitted to pray [[Arvit]] early even if one doesn’t do so during the week. [The same implication could be made from the Tur 267:1.] S”A codifies this as halacha that one is permitted to pray [[Arvit]] earlier than one does during the week.
* [Magan Avraham 267:1 at first questions this ruling of S”A because the Gemara [[Brachot]] 27a seems to assign Rav to the opinion of Rabbi Yehuda meaning that one may only pray early on Friday night if one always follows Rabbi Yehuda. He adds that this is also implied from Kesef Mishna ([[Tefillah]] 3:7) who says that Friday night has the same status as the rest of the week. However, the Magan Avraham answers that the Gemara only meant to assign Rav to Rabbi Yehuda’s opinion in the initial discussion but in conclusion the Gemara reinterpreted the story of Rav to mean that one may pray [[Arvit]] earlier than one does during the week. Additionally he explains that [[Arvit]] was instituted corresponding to the limbs and fats burned at night, however, on Friday night the limbs and fats were not burnt after nightfall. Mishna Brurah 267:3 adds that the ruling of S”A is based on those who say that accepting [[Shabbat]] makes it considered as if it was night regarding [[prayer]].]
* Nonetheless, the Mishna Brurah 267:3 writes that if one follows Rabbi Yehuda on Friday night (and prays [[Arvit]] early) one should make sure to say [[Mincha]] before Plag [[Mincha]] (ten and three quarter hours) so that one doesn’t contradict oneself within one night. The Beiur Halacha (267:2 s.v. VeBePlag) writes that even though there is a lenient opinion which permits a [[minyan]] to pray both [[Mincha]] and [[Arvit]] between Plag [[Mincha]] and nighttime, since the minhag isn’t to rely on this opinion during the week because this opinion isn’t supported by the Gemara on Friday night as well one shouldn’t rely on this opinion. The Mishna Brurah 267:3 writes that the only time to rely on this lenient opinion is in a need of great need and if one prays [[Arvit]] during the time of [[
Ben HaShemashot]]. Yalkut Yosef ([[Shabbat]], vol 1, pg 204-5) writes that one shouldn't contradict oneself to pray [[Mincha]] after plag [[Mincha]] and [[Mariv/Arvit|Arvit]] before nighttime. Nonetheless, Menuchat Ahava (vol 1, chap 6, note 6) writes that it is completely permitted but the minhag is to pray [[Mariv/Arvit|Arvit]] after nightfall.
* [http://www.bknw.org/library/articles/shabbos/Making%20Early%20Shabbos.pdf Rabbi Aryeh Lebowitz] quotes Rav Soleveitchik who held like the Mishna Brurah and concluded that one should pray [[Mincha]] privately without a [[minyan]] before Plag [[Mincha]] so as not to contradict oneself. He also writes that in order to avoid issues of Yuhara (appearing religiously arrogant) one should go to [[Mincha]] when the [[minyan]] is praying and pretend to pray along with them. </ref>
# Only after ten and three quarter hours (Plag [[Mincha]]) one may light [[Shabbat]] candles, and then accept [[Shabbat]] with saying [[Arvit]]. <Ref>S”A 267:2