Gemara

(Mishnah): We say Keri'as Shema at night from the time when Kohanim may eat Terumah.

26a (Mishnah): One may pray Minchah until evening;

R. Yehudah allows until Plag ha'Minchah.

27a (Rav Chisda): Rav used to pray Ma'ariv of Shabbos on Erev Shabbos before dark. This shows that the Halachah follows R. Yehudah. (The time for Ma'ariv begins right after the latest time for Minchah.)

Rav Huna and Rabanan used to wait until dark to pray Ma'ariv. This shows that the Halachah follows Chachamim!

Conclusion: Since the Halachah was not fixed like either opinion, one may follow either one.

Shabbos 118b (R. Yosi): May my portion be among those who bring in Shabbos early in Tiverya (it is in a valley, it gets dark very early).

Rishonim

Rambam (Hilchos Keri'as Shema 1:9): The Mitzvah of Keri'as Shema at night is from Tzeis ha'Kochavim.

Rambam (3:7): One may pray Ma'ariv on Erev Shabbos before sunset. Similarly, one may pray Ma'ariv on Motza'ei Shabbos during Shabbos. Since Ma'ariv is optional, we are not particular about the time, as long as he recites Keri'as Shema after Tzeis ha'Kochavim.

Rebuttal (Ra'avad): One should do so (pray Ma'ariv during the day) only if needed at the time. One must be Somech Ge'ulah l'Tefilah (pray Shemoneh Esreh after Keri'as Shema)!

Kesef Mishneh: The Rambam holds that one may pray Ma'ariv during the day also on weekdays. He cited the Gemara, which discusses Erev Shabbos and Motza'ei Shabbos.

Lechem Mishneh: Why did the Rambam say that the reason is because Ma'ariv is Reshus? The Gemara said that it is because we hold like R. Yehudah, that Minchah is until Plag ha'Minchah, and afterwards is the time for Ma'ariv! The Rambam came to answer why Chachamim did not fix the time for Ma'ariv after Tzeis ha'Kochavim, just like they fixed a time for Shacharis. He answered that Ma'ariv is Reshus. However, Shmuel holds that Ma'ariv is Chovah (obligatory), and he permits praying Ma'ariv of Motza'ei Shabbos on Shabbos! We must say that the Rambam had the Rif's text, in which Abaye and Rava (and not Rav and Shmuel) argue about whether Ma'ariv is Reshus or Chovah.

Question: If it depends on whether Ma'ariv is Reshus or Chovah, how did the Gemara infer from Rav Huna and Rabanan, who waited until dark to pray, that the Halachah follows Chachamim? Perhaps they hold like R. Yehudah, but they hold that Ma'ariv is Chovah!

Answer (Lechem Mishneh): The Rambam did not need to say that Ma'ariv is Reshus to explain why one may pray Ma'ariv of Shabbos on Erev Shabbos. This is a Mitzvah due to Kabalas Shabbos. It is permitted even if Ma'ariv is Chovah. He needed to say that Ma'ariv is Reshus to explain Reshus to explain why one may pray Ma'ariv of Motza'ei Shabbos on Shabbos.

Rosh (1:1): Rashi says that we are not Yotzei Keri'as Shema in the Beis ha'Keneses during the day. We say it only to enter Shemoneh Esreh amidst Divrei Torah. One is Yotzei Shema through the first paragraph during Keri'as Shema Al ha'Mitah. The Yerushalmi says so. The Riva, Ritz Gei'us and Rav Amram Gaon agreed. R. Tam says that we are Yotzei in the Beis ha'Keneses, we hold like R. Yehudah. The Gemara said that one who did like either opinion was fine. This is difficult. The Gemara said so about Tefilah, which corresponds to Korbanos. Minchah was never offered after Plag ha'Minchah! Rather, we hold like other Tana'im who hold that the time for Shema is before Tzeis ha'Kochavim. We are lenient like R. Yehudah regarding Tefilah because it would be hard to gather people again. L'Chatchilah, one should say Shema after Tzeis ha'Kochavim, like our Stam Mishnah. Rav Hai Gaon said that in Eretz Yisrael we pray Ma'ariv, and afterwards we say Shema in its time, even though we lose Semichus Ge'ulah l'Tefilah. However, he must pray Minchah before Plag ha'Minchah.

Rosh (4:6): R. Yoshiyah would pray Ma'ariv on Motza'ei Shabbos during Shabbos. Rav used to pray Ma'ariv of Shabbos on Erev Shabbos, i.e. after Plag ha'Minchah. After Tzeis ha'Kochavim he would say Shema in the proper time. Even though he was not sglt, since he intended for the Mitzvah to add from Chol to Kodesh, he was not concerned for Semichah.

Rosh: All the Amora'im and Tana'im who would pray Ma'ariv of Motza'ei Shabbos on Shabbos and say Havdalah over a cup of wine, they hold like R. Yehudah. Perhaps due to Ones they would not be able to say Havdalah on Motza'ei Shabbos, e.g. they needed to go to the end of the Techum at the end of Shabbos for the needs of a Mitzvah. Surely they would not rush to say Havdalah and pray Ma'ariv if not for an Ones.

Poskim

Shulchan Aruch (OC 267:2): On Shabbos we pray Ma'ariv earlier than on a weekday. From Plag ha'Minchah one may light the Neros, accept Shabbos through Ma'ariv and eat immediately.

Beis Yosef (DH b'Siman): The Tur (293) brings from the Ritz Gei'us that since all of Yisrael follow Rabanan and pray Ma'ariv only after dark, one may not pray Ma'ariv on Erev Shabbos or on Motza'ei Shabbos until the day enters. Since the Rambam and Rosh agree (and permit), we hold like them. Ohel Mo'ed permits after 10 and three quarter hours. This is primary.

Magen Avraham (1): The Gemara inferred that Rav holds like R. Yehudah, because he prayed Ma'ariv of Shabbos during the day. This shows that Chachamim forbid! Also the Rosh and Shiltei ha'Giborim (18b:5) say that all those who prayed during the day hold like R. Yehudah. The Kesef Mishneh says that the Rambam equates weekdays and Shabbos. This is unlike he wrote in the Beis Yosef! R. Yonah connotes that on Erev Shabbos one may pray earlier. On Motza'ei Shabbos, we permit only for a Mitzvah. This requires investigation, for it is unlike the Gemara! I later found that Bahag says 'Rav prayed Ma'ariv of Shabbos on Erev Shabbos. R. Yirmeyah prayed a weekday Tefilah after him, and Rav did not stop him. This shows that Rav permits praying Minchah even after Plag ha'Minchah. If not, he would have stopped him! Rav holds that one may pray Ma'ariv during the day, for he adds from Chol to Kodesh.'

Magen Avraham (1): Bahag must hold that initially the Gemara inferred that Rav holds like R. Yehudah, but it concluded that one may pray Ma'ariv on Erev Shabbos, even if during the week he follows Chachamim. One who investigates will see that the Rambam holds like this, unlike the Kesef Mishneh. I explain that Ma'ariv was enacted corresponding to limbs and fats that are burned on the Mizbe'ach at night. On Shabbos, one may not put limbs and fats of weekday Korbanos on the Mizbe'ach. Also, the Mordechai permits to sometimes follow Chachamim, and sometimes R. Yehudah. We do not hold like this, but on Erev Shabbos one may rely on him. However, the Mordechai (Mo'ed Katan 923) wrote that on one day, one should not do contradictory leniencies. Therefore, on Erev Shabbos one must pray Minchah before Plag ha'Minchah. All this is for one who normally prays Ma'ariv at night. The Ari Zal prayed Ma'ariv at night even on Erev Shabbos. Olas Shabbos says that Chachamim agree about Erev Shabbos. He overlooked the Gemara.

Kaf ha'Chayim (3): The Ari Zal says that Kabalas Shabbos should be at Shki'ah. This implies that Ma'ariv should be at night. Whether one followed the Shulchan Aruch or Ari Zal, this was fine.

Mishnah Berurah (1): The custom was to accept Shabbos through saying Borchu, therefore it was proper to pray early and accept Shabbos early. Similarly, one may accept Shabbos before Ma'ariv.

Yad Efrayim (DH v'Chol): Some are lenient to pray Ma'ariv right after Minchah even during the week, even though both of them are after Plag ha'Minchah. They hold like Chachamim (of R. Yehudah). They are lenient because for it is difficult to gather people twice (for Minchah, and again for Ma'ariv). They rely on the ruling that Ma'ariv is Reshus.

Mishnah Berurah (3): One who prays Ma'ariv during the day must pray Minchah before Plag ha'Minchah. Some allow a Tzibur to pray Ma'ariv during the day, even if Minchah was after Plag ha'Minchah. One should rely on this only if Ma'ariv is at least Bein ha'Shemashos, and in pressed circumstances, but not if it is Vadai day. He repeats Keri'as Shema when it is Vadai night.

Bi'ur Halachah (DH uvi'Plag): Derech ha'Chayim is lenient for a Tzibur, since in other places we are lenient about this also during the week, for it is difficult to gather people twice. In any case one may rely on this for Erev Shabbos. All the Acharonim did not mention this leniency. During the week we pray in the proper time, since this leniency has no source in the Gemara, so we should not rely on it for Shabbos. During Bein ha'Shemashos one may be lenient in pressed circumstances. See the Magen Avraham (489:6) regarding Sefiras ha'Omer (he says that one who counted Bein ha'Shemashos was Yotzei, since it is mid'Rabanan), and the Bi'ur Halachah (261 DH Shi'ur. He says that perhaps one may be lenient only in Bein ha'Shemashos of R. Yosi, but not in Bein ha'Shemashos of R. Yehudah, which is earlier.)

She'elas Rav (ha'Gaon R. C. Kanievsky Shlita, 2:21:11): Also Chazaras ha'Shatz must be before Plag ha'Minchah.

Igros Moshe (2:60 DH Al (2)): One should forfeit Tefilah b'Tzibur in order to pray after dark only if he always follows the Gra. However, if Ma'ariv is a time when people often pray Minchah, he should wait until dark. Even one who prays Shemoneh Esreh with the Tzibur should say Shema and its Brachos only after dark.

Teshuvos v'Hanhagos (2:51,69): One may pray Ma'ariv right after Shki'ah, even if he prayed Minchah after Plag ha'Minchah. Where Shabbos begins late in summer, being stringent can lead to Tza'ar Ba'alei Chayim (hungry children) and arguments with one's wife.

Ashrei ha'Ish (2:5:23, citing ha'Gaon R. Y.S. Elyashiv Ztz"l): Such concerns can justify praying early, but one should make all efforts to avoid this. Ashkenazim in Eretz Yisrael should not make an early Minyan. One who must pray early can pray with Sefardim, or alone.

Mishnah Berurah (4): Before Plag ha'Minchah, one may not light the Neros or accept Shabbos or pray Ma'ariv. If he did, he must pray again. Some (Shulchan Aruch 263:4) say that Plag ha'Minchah is one and a quarter hours before night (Tzeis ha'Kochavim), and some (the Levush and Gra) say that it is one and a quarter hours before Shki'ah.