70:1 The time to recite Shema at night begins when three small stars appear. If the night is cloudy, one should wait until there is no doubt that it’s late enough. The current practice is to daven Maariv immediately following Mincha. Even though it is not yet night, this saves the congregation the effort of having to assemble a minyan a second time. This is only permitted after plag hamincha; before then, one would not fulfill his obligation, even after the fact. (Even after plag hamincha, this is only permitted if one said Mincha before plag hamincha – Mishnah Brurah 235:14.) The Kitzur Shulchan Aruch praises a person who say Maariv at night, with a minyan. One should occupy himself with Torah study in between Mincha and Maariv in order to connect day and night through Torah. If a person davened Maariv with a minyan before night fell, it is still preferable not to eat until after it becomes night. (Mishnah Brurah 235:19 cites authorities that permit one to eat before repeating Shema but still advises that we act stringently in the matter.) Rather, he should wait until the stars come out, then immediately recite the three paragraphs of Shema. (He should not rely upon the Shema he recites upon retiring – MB 235:12.) A person may only say Maariv before the stars appear if he is davening with a minyan.
70:2 One should try to daven Maariv immediately after the stars come out. One may not eat or begin any other activity – not even Torah study – half an hour before the stars appear, just as one may not begin such activities before Mincha. (One may not take a nap at this time. We said in 70:1 that one should study Torah between Mincha and Maariv; however, once the stars appear, one who is going to daven on his own should not study - Mishnah Brurah 235:17.) A person who is committed to studying Torah with a group should not delay saying Maariv any later than midnight; after the fact, one may say Maariv even after midnight, until first light.