Gemara

4b (Beraisa): Chachamim made a protective fence, lest a person come home from the field and say, "I will eat and sleep a bit, and later I will say Shma and pray" -- perhaps he will sleep the entire night!

Rather, when a person comes from the field, he should go to the Beis ha'Keneses and learn. In the proper time, he reads Shma and prays. Afterwards, he will eat his bread and bless;

Anyone who transgresses Chachamim's words is Chayav Misah.

The Tana taught the Chiyuv Misah regarding this decree, because people are often overcome by sleep. Alternatively, he holds that Ma'ariv is mandatory, and teaches unlike the opinion that it is optional.

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.

27b (Shmuel): One may pray Ma'ariv of Shabbos on Erev Shabbos and say Kidush over wine (before evening).

Rishonim

Rambam (Hilchos Shabbos 29:11): One may say Kidush over a cup of wine on Erev Shabbos during the day, even though it is not yet Shabbos. Similarly, one may say Havdalah over a cup during the day, even though it is still Shabbos, since the Mitzvah is Zechirah (mentioning) at the beginning and end, slightly before or after this time.

Magid Mishneh: Even if one said Kidush or Havdalah during the day, he may eat or do Melachah based on this at night. Some Ge'onim and Acharonim say so, this is like the Rambam. This is primary.

Rosh (Berachos 1:9): One may not eat or sleep (some texts - drink) before saying Kri'as Shma at night.

Rosh: We hold like R. Yehudah regarding Tefilah. R. Yakov of Korvil said that one should not eat Matzah (on Seder night) until dark. A Tosefta (Pesachim 2:22) says that Hillel would eat Pesach, Matzah and Maror after dark, one must eat Pesach "ba'Laylah", and Matzah is equated to Pesach. If one ate during the day he was not Yotzei.

Mordechai (Megilah 798): R. Yehudah holds that the Torah exempts a blind person from Mitzvos. He is obligated only mid'Rabanan. How can he be Motzi (those obligated) mid'Oraisa? R. Tuvya of Vina answered that we find that one can say Kidush one and a quarter hours before night on Erev Shabbos.

Suggestion: This is because Tosefes Shabbos is mid'Oraisa, for we learn from Yom Kipur.

Rejection (Mordechai ibid., citing R. Tuvya): R. Tam explained that we learn from "Ad Erev", i.e. only at the end of the day. R. Yishmael holds that the Tosefes beforehand is only mid'Rabanan. Even so, through Kidush mid'Rabanan during the day, after dark one is Yotzei the Torah Mitzvah. Likewise, a blind person is Motzi seeing people who are obligated mid'Oraisa. We say that if women are obligated only mid'Rabanan, they cannot be Motzi those obligated mid'Oraisa. Women are different. They will never become liable mid'Oraisa. A blind person can become liable if he will be cured. One who ate a k'Zayis is obligated to say Birkas ha'Mazon only mid'Rabanan. Even so, R. Yochanan said that he can be Motzi others who ate to satiation and are obligated mid'Oraisa, since he could eat to satiation and be obligated mid'Oraisa. I reject this proof. There, he could obligate himself. A blind person cannot obligate himself. There is a proof from Kidush during the day. He cannot obligate himself mid'Oraisa at the time. He is like one who says Kidush in the middle of the week for the sake of Shabbos.

Magen Avraham (267:1): The Magid Mishneh says that Kidush or Havdalah during the day permits eating or Melachah at night. The Beis Yosef brings so from the Rashba. The Mordechai says that even according to the opinion that Tosefes Shabbos is mid'Rabanan, one is Yotzei through this Kidush after dark, since when he was Mekadesh, he will come to a Torah Chiyuv. This is difficult, for a minor is not Motzi an adult, even though the child will come to a Torah Chiyuv!

Mishbetzos Zahav (OC 273:4): We learn from the Mordechai that Tosefes Shabbos applies before Shabbos from a tradition (from Sinai), and from Shevi'is. Even though we do not learn a Kal va'Chomer from a tradition, we can say that it is a mere Giluy Milsa (reveals) that every Kedushah requires Tosefes. We also learn that Kidush mid'Rabanan is Motzi the Chiyuv Kidush mid'Oraisa. Since the primary Mitzvah is Zechirah (mentioning), this is enough, like the Mordechai says. Likewise, one who mentioned Shabbos in Tefilah on Shabbos night ("va'Ychulu...") can be Motzi in Kidush one who did not pray. However, there he can be Motzi in any case due to Arvus. However, we can say that a minor cannot be Motzi someone obligated mid'Oraisa.

R. Akiva Eiger (2): The Kesef Mishneh (Hilchos Korban Pesach 5:7) is unlike the Mordechai. (He said that if we offered Korban Pesach for a minor, he is exempt from Pesach even if he matured in between. This is because the Torah taught that we slaughter for him and give him a share. According to the Mordechai, we need not say so! - PF) The Mishbetzos Zahav says that the Mordechai exempts because the primary Mitzvah is Zechirah. He did not see the Mordechai, who explicitly says like the Magen Avraham brought. He did not explain his reason. I challenge what he learned to a blind person, for in Tosefes Shabbos surely he will come to a Torah Chiyuv, since the time comes automatically. The Mishneh l'Melech (Hilchos Ishus 4:7) made such a distinction regarding Davar she'Lo Ba l'Olam.

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, or say Kidush or Havdalah, until the day enters. This is whether or not wine is available, for one may say Kidush on bread. Since the Rambam and Rosh agree (and permit), we hold like them. Ohel Mo'ed permits Ma'ariv, Kidush and Havdalah after 10 and three quarter hours. This is primary, even though some Ge'onim forbid Kidush and Havdalah until it is dark.

Beis Yosef (DH u'Mah she'Chasav Rak): Mahari Avuhav permits one who accepts Shabbos early to eat immediately. The Rosh connotes that only Matzah must be eaten at night, but one may eat on Shabbos before the time. R. Yerucham (1:66:3) agrees. Why is this permitted? The Gemara forbids eating and drinking before saying Shma! Some Poskim say that the time for Shma is from Plag ha'Minchah, like the Rosh brings (1:1). Therefore, even though we repeat Shma after Tzeis ha'Kochavim due to the Poskim who say that then is the time, this is not called eating before Kri'as Shma.

Magen Avraham (2): The Beis Yosef overlooked the Rosh, who says that Rav said Kri'as Shma after Tzeis ha'Kochavim, and he was not Somech Ge'ulah l'Tefilah (pray Shemoneh Esreh after Kri'as Shma)! He ate before Kri'as Shma, i.e. he began eating half an hour before the time. Also Divrei Chamudos (Berachos 4:29) says so. Those who pray Ma'ariv on Erev Shabbos may not begin to eat half an hour before the time for Kri'as Shma, for they were not Yotzei at all through Kri'as Shma during the day.

Kaf ha'Chayim (7): In Siman 235, the Magen Avraham was lenient like the Beis Yosef (to permit beginning within half an hour of night, since he already said Kri'as Shma during the day). We must say that here he discusses those who pray Ma'ariv before Plag ha'Minchah.

Gra (DH v'Le'echol): The Gemara said that he may (immediately) say Kedushah (i.e. Kidush. Shmuel permitted this, and he holds that Kidush is only where one eats - Damesek Eliezer.)

Mishnah Berurah (6): If after praying Ma'ariv there is less than half an hour until night, one may not start eating. He waits until night and repeats Shma without the Berachos, and then eats. Many Poskim say that one is not Yotzei Shma before night. However, the custom is to be lenient about this, and we do not protest, for he has whom to rely on. This is regarding eating. Surely one must repeat Shma at night to fulfill the Mitzvah.

Magen Avraham (1): The Shlah, and the Bach citing Maharal, say that one must eat a k'Zayis at night, to fulfill three meals on Shabbos. Tosfos and the Rosh connote that one may finish the meal during the day, and so say Terumas ha'Deshen (1) and R. Yerucham. This is because after Plag ha'Minchah, it is considered night. In any case, it is good to be stringent.

R. Akiva Eiger (1): If Reuven did not accept Shabbos, I am unsure whether he may be Motzi in Kidush Ploni, who accepted Shabbos. For Reuven it is a weekday, so it is as if he is not commanded about Kidush now. This is like a city dweller (who reads the Megilah on the 14th. He cannot be Motzi villagers, who read before this.) Or, perhaps here is different, for Reuven can accept Shabbos now and become obligated. This requires investigation.

Mishnah Berurah (5): Since he accepted Shabbos and added from Chol to Kodesh, it is considered Shabbos. He may say Kidush and eat immediately and finish his meal during the day. Some say that he must continue the meal until night and eat a k'Zayis then, for we learn the three meals of Shabbos from the three times it says "day" regarding eating the manna on Shabbos. Therefore, he must eat them during the day itself. L'Chatchilah, one should be concerned for their opinion.

Kaf ha'Chayim (6): The Taz (291:7) says that Tosefes Shabbos is mid'Oraisa. Therefore, one need not eat after dark. However, some say that it is mid'Rabanan, and some say that even mid'Rabanan there is no Tosefes Shabbos. Therefore, it is good to be stringent.