The gemara (Brachos 49a) writes that if a person forgot to recite ya’aleh v’yavo in bentching on Yom Tov but did not yet start the bracha of hatov v’hameitiv, he/she inserts the bracha “Baruch shanasan Yamim Tovim l’amo Yisrael l’simcha ul’zikaron…”. If the same mistake is made on Rosh Chodesh, a similar bracha is recited, “Baruch shanasan Roshei Chodashim l’amo Yisrael l’zikaron…”, but here the gemara quotes R’ Zeira as being in doubt whether the text includes the word “simcha” or not.
Food for thought: what is the gemara’s safeik? Was the gemara entertaining the possibility that there is a din of simcha on Rosh Chodesh like on Y”T (if so, why is there no chiyuv seudah?), or is the gemara’s safeik simply whether the lower level simcha of Rosh Chodesh warrants mention in the bracha?
It is a bit ironic to post this on Rosh Chodesh Av, as mshe’nichnas Av m’ma’atim b’simcha, but as seforim explain, the greatest ohr comes from the removal of hester panim; with the geula shleima which we will hopefully soon see these weeks of bein hametzarim will be the “chol hamoed” between the great yamim tovim of 17 Tamuz and 9 Av.