Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 580…
“ Shulchan Aruch Orach Chaim 580 Fast days ימים שמתענים בהם 580:1 These are days that tragedies happened to our ancestors and are therefore fitting for fasts. Even though some of them fall on Rosh Hodesh, there is a position that one should fast on them (though it is good to avoid completing a fast on Rosh Hodesh) . ”
