Siman 421

It is the custom in Ashkenaz to say the parasha of "And on your New Moons" during Shacharit after the parasha of the Tamid. In Sefarad, they do not practice saying it, because they are destined to read it in the Sefer Torah. Specifically on Shabbat they say the verses of "And on the Shabbat day," because they do not read it on Shabbat in the Torah. This is not the case for Rosh Chodesh, which is similar to Yom Tov, for they do not say the Musafim of Yom Tov after the Tamid, because they read them in the Torah. Here too regarding Rosh Chodesh. The reasoning of Ashkenaz is in order to publicize that it is Rosh Chodesh. For Yom Tov is recognized from yesterday that it is Yom Tov, but Rosh Chodesh is not recognized at all, and they might forget to say "Ya'aleh V'Yavo" in the prayer. Therefore, they remind everyone by saying the verses before the prayer (see Magen Avraham).