When is Plag Hamincha?
Plag Hamincha is 1 and ¼ Zmaniyos hours prior to sunset.
 Ketzos Hashulchan 76/1. This is based on Siddur Hilchos Kerias Shema and 443/4 which rules that the hours of day are calculated from sunrise to sunset, and so is the custom today of all Jewry. In accordance to this calculation Plag Hamincha is 1 and ¼ hours prior to sunset, and one may even light the Shabbos candles from that point. This is in contrast to the ruling here in 261/5 where Admur rules that although one may be stringent to accept Shabbos from 1 and ¼ hours prior to sunset, he may not be lenient to light candles until 1 and ¼ hours prior to nightfall.
Other Opinions: Some opinions rule that Plag Hamincha is 1 hour and 15 minutes before nightfall. [M”B 692/13; Kaf Hachaim 692/29]
Ruling of Admur in Shulchan Aruch: In 263/6 Admur rules that Plag Hamincha is 1 hour and 15 minutes before nightfall. This is based on 58/3; 89/1 in which Admur rules the day is from Alos until Tzeis. Likewise in 261/5 where Admur rules that although one may be stringent to accept Shabbos from 1 and ¼ hours prior to sunset, he may not be lenient to light candles until 1 and ¼ hours prior to nightfall. However in Admur 443/4 he rules it is counted from sunrise until sunset and so rules Admur in the Siddur.
The following is the ruling of Admur brought in Shulchan Aruch here [261/5; see also 58/3; 89/1 which rules the day is calculated from Alos-nightfall based on M”A 261/9]: Admur here rules that Plag Hamincha is 1 and 1/4 hours prior to nightfall. Nevertheless he concludes that regarding accepting Shabbos one is to be stringent from 1 and ¼ hours prior to sunset to not do any Melacha from that point and on if he accepted Shabbos then, being that there are opinions [Yereiim] which state that Plag Hamincha begins 1 and ¼ hours prior to sunset. [As they hold that night begins immediately after sunset, while Bein Hashmashos begins before sunset and last ¾ of a Mil.] Accordingly there are those which are meticulous to initially separate from Melacha like this latter opinion starting 1 and ¼ hours prior to sunset. Nevertheless even one who chooses to accept Shabbos from 1 and ¼ hours before sunset may not yet light candles, or Daven Maariv of Shabbos, [or say Kiddush] until 1 and ¼ hours prior to nightfall, as according to the first opinion, which is also the main opinion, these acts cannot be done until 1 and ¼ hours before nightfall. Furthermore if one went ahead and lit candles or Davened Maariv within 1 and ¼ hours prior to sunset, but before 1 and ¼ hours prior to nightfall, he must extinguish and relight the candles, as well as repeat the Davening, later within the proper time, as the main opinion is like the first opinion mentioned. [ibid]
The above however is only in accordance to this ruling of the Shulchan Aruch. However in accordance to the Siddur in which we calculate the day from sunrise to sunset, Plag Hamincha is also only calculated from before sunset and one may therefore fully accept Shabbos from that point and on, including lighting candles with a blessing, and so is the final ruling and custom of all Jewry.
In conclusion all agree that one may accept Shabbos 1 and ¼ hours prior to sunset. The dispute is only regarding if one may light candles then, and the final ruling follows that one may.
- Tzaruch Iyun: Here in 261/5 Admur rules regarding Plag Hamincha that we are Machmir like Geonim/Yireim, and that there are people which are accustomed to follow Geonim/Yireim even initially. However in 331/5 Admur rules that there is no need to suspect for the opinion of the Yireim/Geonim regarding doing Mila on Shabbos, when the child was born after sunset the previous Shabbos. His reasoning is because there are no local communities which are stringent like the geonim. Hence there is a contradiction as earlier he ruled to suspect for Geonim/Yereim even regarding Tosefes Shabbos, certainly then for Shabbos itself one should suspect. Furthermore earlier he mentioned that there are people which initially are stringent like Rabbeinu Tam, and here he mentions that there are no communities which are.
Answer: Admur admits that there are people which follow the ruling of Yereim/Geonim, but in our provinces they don’t at all, and hence regarding Milah Bezmana, we will not push it off just because of a Chashash. However regarding following the Geonim on Erev Shabbos there is nothing to lose by being Machmir, and hence one should do so.
Thus in conclusion, Admur does suspect for the opinion of the Yereim/Geonim, and it is only in case of pushing off mila that he says in such a case one need not be stringent, which leads to a leniency in Mila. However regularly one is to be stringent.
However tzaruch Iyun Gadol, as accordingly why does Admur not rule in Shulchan Aruch to be stringent like the Yereim/Geonim, and only in the Siddur does he rule to do so [like the Geonim, however regarding the Yireim, even in the Siddur he does not rule to be stringent]. Furthermore, if regarding Tosefes Shabbos Admur rules to be stringent, then certainly regarding Shabbos itself he should rule that way.
Perhaps however one can answer that in truth Admur does not suspect at all for Yireim/Geonim, as the custom is to be completely lenient, and hence the custom rules like Rabbeinu Tam, and there is no need to be stringent, as explains Admur in 331. However if one goes ahead and davens Maariv at the time that only the Yereim/Geonim allow, then he has done an action which shows that he follows the Geonim, and hence in such a case he should be stringent like the opinion that he physically chose to follow, However regularly there is no need to suspect for their opinion.