Moed Katan 3:5

If a person buried a deceased relative three days before yom tov, shiva (the seven-day mourning period) is canceled for him; if eight days before yom tov, shloshim (the 30-day mourning period) is canceled for him. This is because Shabbos counts as a day of mourning and does not interrupt a mourning period while yom tov interrupts a mourning period and does not count as a day of mourning.

Moed Katan 3:6

Rabbi Eliezer says that since the Temple was destroyed, Shavuos is like Shabbos when it comes to the effect on mourning periods. Rabban Gamliel says that Rosh Hashana and Yom Kippur are like yom tov in this matter. The Sages, however, disagree with both of these statements; they maintain that Shavuos is like yom tov, while Rosh Hashana and Yom Kippur are like Shabbos.