If someone dies just before or between Rosh Hashana and Yom Kippur is it correct to sit Shiva and when should you do it?

A "Reggel" = Jewish biblical festival, has the power of annulling the Shivah – the mourning period, as long as the mourner had sat Shivah for some time prior to the Chag. If he did not for any reason, or if the relative passed away on the Chag itself the Shivah starts after the Chag. Therefore for instance, when the relative dies the day before Rosh Hashanah, he stops his Shivah after one day. More so, the next Chag will annul the "Sheloshim" period of mourning. So in the above case Yom Kippur will end the Sheloshim.