219:7 Even though shiva is not observed on yom tov or chol hamoed, the rules of shloshim do not take effect on these days and the mourner may wear freshly-laundered clothes. Since one may not shave over the course of a holiday anyway, it counts towards the thirty days of shloshim, so he counts thirty days from the burial. Even though Shemini Atzeres is an independent holiday, since he hasn’t started mourning yet, it doesn’t cancel mourning. (See 220:1 for how holidays cancel the restrictions of shiva and shloshim.) Similarly, it only counts for one day of the shloshim.
219:8 Regarding a newlywed who is also a mourner, if they were married before the holiday, and the holiday fell during his week of sheva brachos, and a relative died during the holiday, then all the days of sheva brachos are not included in the count of thirty days for shloshim.