220:6 If the mourner didn’t shave before Shabbos or yom tov, he is not permitted to shave on chol hamoed because he could have done so earlier, but he is permitted to shave after the holiday. If the last day of shiva will be a Shabbos that is also erev yom tov, since he was permitted to shave according to the laws of mourning but Shabbos prevented him from doing so, it is considered a circumstance beyond his control and the mourner may shave on chol hamoed
220:7 Yom tov cancels the restrictions of shloshim but only for relatives other than parents. For a parent, however, one may not shave until his friends rebuke him for his unkempt appearance. Therefore, yom tov does not cancel this restriction.