Early Rosh Chodesh

Question

There's a concept of early Shabbos. A person can accept Shabbos early, pray Shabbos Maariv before dark (according to Rabbi Yehudah, who says you can pray after Plag HaMincha; I admit this one is less unanimous), say Kiddush, are prohibited in melachos, etc. The same is true for Yom Tov (with a few exceptions).

Is there such a concept as early Rosh Chodesh? I'm not sure how'd they'd accept it upon themselves. Perhaps by praying Maariv after Plag HaMincha? If someone prays Maariv after Plag HaMincha on Erev Rosh Chodesh, do they say Yaaleh VeYavo? If they then eat a bread meal, do they mention it in Bentching? Those who are accustomed to refrain from certain activities on Rosh Chodesh, would they then apply?

Answer 1

As much as I wish we could have tosefes Rosh Chodesh, Rav Doniel Schreiber writes quotes the Magen Avraham (OC 419:1) saying that there is no such thing. He continues by quoting the Mishnas Ya'avetz (OC 12) explaining that the reason we can say Ya'aleh Veyavo during an earlier Ma'ariv is because the tefillah relates to the next day and is enough to justify saying the Rosh Chodesh tefillah. Additionally, Rav Yaakov Goldstein quotes the Shulchan Aruch Harav (188:17) who agrees, saying

וְיֵשׁ אוֹמְרִים שֶׁבְּרֹאשׁ חֹדֶשׁ חֲנֻכָּה וּפוּרִים הוֹאִיל וְאֵין בָּהֶם תּוֹסֶפֶת מֵחֹל עַל הַקֹּדֶשׁ אֵינוֹ מַזְכִּיר מֵעֵין הַמְּאֹרָע אַחַר שֶׁיָּצָא הַיּוֹם. וְאֵין נוֹהֲגִין כֵּן. - There are authorities who maintain that since there is no concept of “adding from the mundane to the holy” on Rosh Chodesh, Chanukah, and Purim, one should not mention the uniqueness of the day after the day has already departed. This is not the accepted custom.

Nonetheless, Rav Yaakov Goldstein also quotes the Lubavitcher Rebbe z"tl (Shulchan Menachem 2:204) saying that tosefes Rosh Chodesh does indeed apply according to the Poskim who hold that there is tosefes kedusha on Tisha B'Av [See Beis Yosef 553].