1,370. Mourning Doesn't Commence on Yom Tov
219:5 If a person buried a relative or received timely notification of a relative's passing on a festival, whether on yom tov or during chol hamoed, he does not start mourning until after the holiday. Things that are done in private are observed as per usual. He doesn't change his clothes because this would be considered mourning in public
Another mourner might change his clothes even on a holiday, but since the person in our example has not yet begun to mourn, he doesn't. A mourner whose practice is to wear tefillin on chol hamoed should do so even on the day after the burial.
219:6 After the holiday, the mourner starts to count the seven days of shiva. The last day of the holiday is included, so he counts six additional days. Even the second day of Rosh Hashana is included in the count.