Actually, your basic question is a very good question! The Olas Shlomo says that there is no Hekesh mentioned in Shas for Se'ir Rosh Chodesh and Regalim. He suggests that since it says regarding Rosh Chodesh and Regalim, "Se'ir Izim l'Chatas," they are compared through a Hekesh to the standard Chatas of a Yachid which needs Shirayim on Mizbach ha'Olah. (See Olas Shlomo who writes more at length on this question.)

All the best,

Reuven Weiner