11. Yom Tov-When to start and end Shiva if Yom Tov coincides:

*See Chapter 27 Halacha 1 and 3-4 for the full details of this subject! To follow is a mere summary:

Shiva began before Yom Tov: If one began sitting Shiva before Yom Tov and Yom Tov then arrived within the Shiva [i.e. Pesach, Shavuos, Sukkos, Rosh Hashanah, and Yom Kippur] then the Shiva becomes nullified by the Yom Tov, and after the Yom Tov one only needs to count the remainder of Shloshim. See Chapter 27 Halacha 1 for the full details of these laws.

Shiva did not begin before Yom Tov: In all cases that the Shiva did not begin before the Holiday and was hence not nullified by the Holiday, then the seven days of Shiva only begins once the Holiday [including Chol Hamoed and the last day/s] is over. This applies whether the burial took place before Yom Tov and one was simply not made aware until Yom Tov, or did not have enough time to begin Shiva, or if the burial took place on Yom Tov/Chol Hamoed. However, in the Diaspora where they keep two days of Yom Tov, the Shiva is counted starting from the last day of Yom Tov, and hence they only count six days of Shiva after Yom Tov concludes. Nonetheless, they do not actually sit Shiva on the last day of Yom Tov, and it is treated like a Yom Tov for all matters. Throughout Yom Tov/Chol Hamoed, the mourners are to keep the discreet laws of mourning that are followed on Shabbos during Shiva. See Chapter 27 Halacha 5A for the full details of these laws. Regarding if the last day of Yom Tov is on Shabbos, or on Erev Shabbos, see Halacha 3!

Buried on last day of Yom Tov: If the deceased was buried by a gentile on the first day/days of Yom Tov, or on Chol Hamoed, then the Shiva does not begin until Yom Tov is over, as stated above. However, if the person died on the second day of Yom Tov in the Diaspora, which is the last day of Yom Tov [i.e. 2nd day of Shavuos, Acharon Shel Pesach, Simchas Torah, with exception to 2nd day of Rosh Hashanah] and the burial took place on that day, then it is disputed amongst Poskim as to whether the relatives who are Biblically required to mourn are to already begin Shiva on that day. Practically, the custom of the world is to never begin sitting Shiva on Yom Tov, even if it is the second day of Yom Tov in the Diaspora, and one of the Biblical relatives died and was buried on that same day. Nonetheless, one does begin counting Shiva from that day, and hence one only practices Shiva for another six days after the Holiday. See Chapter 27 Halacha 5A for the full details of these laws.