Melacha on Rosh Chodesh

Question

Chodesh Tov Rav! Where does the Rama in the Shulchan Aruch say that if Rosh Chodesh falls on Erev Shabbat, one cannot shave prior to shabbat? Why is this opposite to when the same situation arises during S'firat Haomer- there, one is permitted to shave Lichvod Shabbat Kodesh? Thanks