MISHNAH: WHICH KORBANOS MAY BE BROUGHT ON YOM TOV
(Beis Shamai): Shelamim are brought on Yom Tov, but not Olos.
But Semichah is not done on them on Yom Tov.
(Beis Hillel): Shelamim and Olos are brought on Yom Tov.
And Semichah is done on them on Yom Tov.
(Beis Shamai): When Shavuos falls on Friday, its Korbanos are brought on Sunday.
(Beis Hillel): When Shavuos falls on Friday, its Korbanos are brought on that day.
Beis Hillel agrees that when Shavuos falls on Shabbos, its Korbanos are brought on Sunday.
But the Kohen does not wear his special garments on Sunday, and eulogies and fasting are permitted, in order to refute the Tzedukim.
THE FIRST SOURCE FOR TASHLUMIN OF KORBANOS OF SHAVUOS
(R. Elazar in the name of R. Oshiya): There is a Hekesh from Shavuos to Pesach (Devarim 16:16), thus giving Shavuos seven days of Tashlumin.
Even though there is also a Hekesh from Shavuos to Sukos (and thus Shavuos should have eight days of Tashlumin), "Tafasta Merubah Lo Tafasta" and that Hekesh is not used to teach Tashlumin for Shavuos.
Question: Why, then, is Sukos included in the verse?
Answer: To teach that Sukos requires Linah just like Pesach.
Question: Beis Hillel in the Mishnah says that when Shavuos falls on Friday, "it has no day to make up the Korbanos."
This implies that Shavuos has no Tashlumin!
Answer: It means that it does not need to make up the Korbanos (since they may be brought on Yom Tov).
Question: A Beraisa says that one who did not bring the Korban of the Chag on "Y. T. of Shavuos" cannot make it up.
This seems to mean "Yom Tov of Shavuos" -- meaning that Shavuos has no Tashlumin!
Answer: "Y. T. of Shavuos" means "Yimei Tavoach" (the days of bringing the Korban, implying that Shavuos does have Tashlumin).
Question: A Beraisa says that Shavuos is celebrated according to the units by which it is counted (i.e. days), implying that it is only one day, and it does not have Tashlumin!
Answer: It is also counted by weeks, and thus it does have Tashlumin for seven days.
This is as Abaye said, "It is a Mitzvah to count the days to Shavuos (as in Vayikra 23:16), and it is a Mitzvah to count the weeks to Shavuos (as in Devarim 16:9).
THE SECOND SOURCE FOR TASHLUMIN OF KORBANOS OF SHAVUOS
(Tana in Yeshivah of R. Elazar ben Yakov): The Torah says that Shavous shall be celebrated "when you reap the harvest of the land" (Vayikra 23:22).
This reaping cannot be referring to the actual day of Shavuos, because reaping is forbidden on Yom Tov!
It must be referring to the days of Tashlumin of Shavuos.
Even though one source was already given by R. Elazar in the name of R. Oshiya (the Hekesh from Shavuos to Pesach), both sources are necessary.
If we only had the first source (the Hekesh), we would think that just like the seven days of Pesach have an Isur of Melachah, so, too, the seven days of Tashlumin for Shavuos; therefore we need the second source ("when you reap").
If we only had the second source ("when you reap"), we would not have known how many days of Tashlumin Shavuos has; therefore we need the first source (the Hekesh).