If I Daven Maariv early on Rosh Chodesh, immediately after Plag HaMincha, do I still say Yaaleh V’Yavo in Birkas HaMazon.
If I Daven Maariv early, immediately after Plag HaMincha, and then change into my pajamas, should I leave my Tzitzis on? If so, until when? Shkia? Tzeis?
The topic you are asking about is talked about a lot in the poskim, and it is not isolated to Rosh Chodesh. It also applies to counting sefirah of the previous day after an early maariv, for a woman to making a hefsek tahara, to sit in the sukkah on Shmini Atzeres, to do a bris after maariv, to put on tzitzis, or teffilin, starting avelus, the correct date in a get, and a number of other places, The question is a general one, what day is it after one davens maariv early, is it considered the next day, since he already davened he tefillah of the next day, or do we say that the time for davening maariv doesn’t essentially change the day, rather one may daven maariv even though it is still day time. (The rationale for this is because tefilas maariv corresponds to the bringing of the aymurim in the Bais Hamikdash, which were brought at night, however they wee allowed to be brought from plag hamincha.)
In essence there is a macklokes rishonim about this. See Terumos Hadeshen 248, that he is of the opinion that after maariv it is considered the next day already, and this is also the opinion of the Maharam. However The Agur (1377) and Maharil (95). The Rema Y:D 196-1, and shach ibid 4, rule like the Maharil, but keeps that preferably we should be careful about it. (Also see Gra Y:D 196-6) therefore the Rema paskens that essentially it is still daytime. Therefore you should definitely leave your tzitzis on until Tzies Hakochavim. Regarding Ya’aleh Vyavo, it would appear that he shouldn’t say it, since halachically we don’t regard it to be the next day yet.
As a side point the Rema O:CH 18-1 says not to make bracha when putting on tzitzis after davening maariv early. See M:B 18-7, regarding making a bracha on tzitzis during bein hashamshos.
Regarding Yaaleh Vyavo- Besides that we paskin that it is still the first day, even if we would consider it a safek, saying yaaleh vyavo when we shouldn’t is worse than not saying it at all.
Additional sources- regarding Milah Y:D 262-7, Regarding Shofar Taz O:CH 600-2, regarding Teffilin O:CH 65-17, (should be put on without a bracha. Regarding aveilus Y:D 402-11, Maharshak O:CH 600 D:H Ach, and Maharsham 3-333. Regarding counting sefirah- O:CH 489-3 Biur Halacha D:H Mbiod Yom, Igros Moshe O:CH 4-99-(3). Regarding a bar mitzvah boy davening early the dy before hiss bar mtzvah, see Mashiv Davar 1-18, Binyan Shlomo O:CH-60, Minchas Yitzchok 8-17.