The Mishnah Berurah writes (418:1)  that the community of Worms in Germany fast on Rosh Chodesh Sivan and recite selichos, to commemorate the evil degrees enacted against the Jews on that date in the year 4856  (1096).  What is the leining on this fast? In the morning one does the leining for Rosh Chodesh and in the afternoon “vayechal”. (Kaf Hachaim 418:2)  Other cases of fasts decreed for Rosh Chodesh can be found in a list of fasts appearing in Shulchan Aruch,  Orach Chaim 580. These fasts include Rosh Chodesh Nisan and Rosh Chodesh Av.  The source of this list can be traced back as far as the Behag (the period of the Geonim) – (it is often incorrectly stated that this list is the last chapter of Megillas Ta’anis).  The fasts on these two dates even have an application today, since a Choson and Kallah fast if they get married on Rosh Chodesh Nisan  and  (at least theoretically) on Rosh Chodesh Av. (Mishnah Berurah 573:9), although not on other Roshei Chodoshim.

The last paragraph of the middle berachah in the amidah for Shabbos begins with “... retzeh bimnuchoseinu, kadsheinu bemitzvosecho ...” The wording in this paragraph is to be found in every Shabbos amidah, even when any Yomtov occurs on Shabbos – with one exception! This is in musaf for Shabbos Rosh Chodesh when this wording for Shabbos is almost entirely absent from the amidah. The Aruch Hashulchan (Orach Chaim 425:2) considers that they were omitted by a printer and he personally would say the words which had been omitted when he said this amidah.

One of today’s leading Poskim and also head of the largest non governmental Beis Din in Israel, Rav Nissim Karelitz of Bnei Brak, each day includes fish in his daily lunch menu. However, on Rosh Chodesh meat is substituted for the fish. (Mishpacha, English edition, 13 April 2011, page 48)