69:8 If the Mincha service lasts until nightfall (that is, until the stars appear - Mishnah Brurah 131:16), Tachanun is not said, because Tachanun is not recited at night. One must be careful that Mincha not go past nightfall because one may not say the full Kaddish at night over prayers that were said during the day. This is because night is already the start of the next day.
69:9 If a person arrives at shul for Mincha on Friday afternoon and finds that the congregation has already accepted Shabbos by reciting Mizmor Shir l'Yom HaShabbos (Psalm 92) - or if they accepted Yom Tov by saying Barchu - then he should not daven Mincha there; he should go outside and say it somewhere else. If he hears the shaliach tzibbur say Barchu, he should not reply with the congregation, and if he does, he can no longer recite the weekday Shemoneh Esrei. If he erred and answered Barchu, he says the Maariv Shemoneh Esrei twice (the second to make up for the missed Mincha). If a person arrived shortly before the congregation accepts Shabbos or yom tov, even though he can't finish Shemoneh Esrei before they accept Shabbos or yom tov, he may nevertheless recite the weekday Mincha because it was permissible at the time he began (though it's still preferable to daven Mincha elsewhere - Mishna Brurah 263:63).