Night Prayers While The Sun Is Shining?!
In the summer months, many synagogues elect to pray the Maariv (evening) prayer before nightfall. Similarly, these communities usher in the Shabbat on Friday afternoon well before the stars appear in the heaven. By praying the Minchah (afternoon) prayer before the “Plag Haminchah,” which is 1¼ halachic1 hours before sunset, it is permitted to pray Maariv and accept the Shabbat any time after the Plag Haminchah. This is especially convenient in places where nightfall in the summer months is quite late; and without the convenience of Plag Haminchah, the Shabbat meal wouldn’t start before 10:00 pm.

The Dispute
The exact times when the Mincha and Maariv services are to be prayed is a subject of disagreement between the Mishnaic sages. Rabbi Judah maintained that Minchah can be prayed (from one half hour after midday) until 1¼ hours before sunset. His rabbinic counterparts argued that one may say the Minchah prayers until the end of the day.

According to the rabbis, the Minchah prayer was instituted in correspondence with the afternoon communal sacrifice, which technically may be offered as long as it was day. Rabbi Judah contended that Mincha corresponds to the offering of the incense, which was offered at least 1¼ hours before sunset.

The Maariv prayer directly follows Minchah. So according to the rabbis Maariv must wait until nightfall, while Rabbi Judah holds that Maariv may be prayed anytime after the Plag Haminchah.

The “Politically Correct” Verdict
In an unusual quirk of Jewish law, this dispute has never been resolved. Instead, one may select whichever opinion is more convenient! You can recite the Maariv after nightfall, or you may pray the Maariv before nightfall, after the Plag Haminchah, provided that you recited Mincha before the Plag Haminchah. 

However, one should not be constantly vacillating between the two opinions; one day like this and the next like that—choose the more convenient option and stick with it. Shabbat is the exception to this rule: because we are eager to be graced by the holiness of the day, even one who during the week always prays Maariv after nightfall may pray the Shabbat evening prayers on Friday afternoon after the Plag Haminchah.

The Shema
Although one can pray the Maariv before nightfall, the Shema must be repeated after the stars appear. Also, when praying before sunset, the counting of the Omer is omitted, and must be said after dark.