When one is mekabel shabbos early and friday is rosh chodesh, should one say yaleh v’yavo in the shabbos davening? Is it best not to make early shabbos so as not to enter into such problems?
If one davens Maariv of Shabbos early on Friday Rosh Chodesh, Yaaleh Veyavo is not said (Magen Avraham 419:1; Mishnah Berurah 423:2). The reasoning is that with regard to Maariv it is considered as night. This principle is based on the Terumas Hadeshen (109), and others.
Note that this is not necessarily true for one who eats before Maariv, accepting Shabbos by making kiddush, and this will probably depend on the dispute between Maharshal and Taz concerning eating early on the night of Shemini Atzeres (does one make the beracha on the Sukkah). It is preferable not to do this when Friday falls on Rosh Chodesh.