He who buries his dead [relative] three days before a festival has the edict of seven [days of mourning] annulled. [He who buries his dead relative] eight [days before a festival] has the edict of thirty [days of mourning] annulled. [This is so] because they said: Sabbath is included [in the period of mourning] but does not interrupt [it]. Festivals [however] interrupt but are not included.

R' Eliezer says: From the time the Temple was destroyed Shavuos is like the Sabbath.  Rabban Gamliel says: Rosh Hashanah and Yom Kippur are like the festivals.  The Sages, however, do not agree with either of the two. Rather, Shavuos is like the festivals; Rosh Hashanah and Yom Kippur are like the Sabbath.