1. The Unique Status of Isru Chag Shavuos
The Alter Rebbe brings in Shulchan Aruch—the Code of Jewish Law—that the Isru Chag of Shavuos has a unique status, being more of a true Yom Tov—festival—from the perspective of halacha. It is prohibited to fast on this day, not merely as a custom but as a matter of law. While it is customary not to fast on any Isru Chag due to its connection with the preceding festival, by Shavuos there is an added dimension: it was known as the Yom Tovuach—the Day of Slaughtering.2. Yom Tovuach and the Bringing of Korbanot
In the times of the Beis HaMikdash, the day after Shavuos was when people would bring their festival offerings, specifically the Olas Re’iyah—the pilgrimage burnt-offering. There is a dispute between Beis Shammai and Beis Hillel regarding whether these offerings could be brought on Yom Tov itself. Although we follow Beis Hillel that it is permitted, the prevailing custom was to bring them on the day after Shavuos. As cited by the Alter Rebbe from the Gemara, many people celebrated this day, making it a special occasion in its own right.3. The Concept of Tashlumim for Shavuos
Just as with Pesach and Sukkos, where offerings could be brought throughout all seven days due to Tashlumim—make-up days—so too with Shavuos. Even though Shavuos itself is only one day, there are seven days for bringing missed offerings. This is why we do not recite Tachanun—supplicatory prayers—through the 12th of Sivan, corresponding to these days of Tashlumim.4. Counting Days: The 12th or 13th of Sivan?
The Rebbe notes that some authorities extend this period through the 13th of Sivan due to uncertainty about which day is truly Shavuos (Mesafek deYoma). If Shavuos falls on the 7th, then seven days would reach until the 13th. However, the Rebbe rules that we only count through the 12th because these days are specifically tied to bringing korbanot in the Beis HaMikdash. Since there are no korbanot outside Eretz Yisrael (Chutz La’aretz), there is no reason to extend beyond seven days from Vav Sivan.5. The Calculation Based on Nisan and Its Implications
It appears logical that since Shavuos always follows 49 days from Pesach, there would be no need for an extra day for korbanot because everyone would know exactly when Shavuos falls based on their knowledge from Nisan. The months leading up—Nisan, Iyar, and Sivan—would provide clarity about when to begin and end these seven days.6. Yom Tov Sheini Shel Galuyot and Its Stringency for Shavuos
A question arises: why do we keep a second day of Yom Tov for Shavuos in Chutz La’aretz? The Rebbe cites both Rambam and Chasam Sofer who explain that Yom Tov Sheini Shel Galuyot—second festival day in exile—is even stricter for Shavuos than other festivals. This is because we do not distinguish between different holidays (L’cholik b’moedim)—since Pesach and Sukkos have two days in exile, so does Shavuos. This idea parallels what we find in Gemara Rosh Hashanah (around daf 21), where Rabbi Yochanan ruled not to differentiate between places reached by messengers in Nisan but not in Tishrei regarding observance of two festival days.7. Application to Korbanot and Isru Chag in Chutz La’aretz
Despite this stringency regarding two festival days, when it comes to korbanot—the sacrificial offerings—the extension does not apply outside Eretz Yisrael since there are no sacrifices brought there. Therefore, while we keep two days of Yom Tov in Chutz La’aretz for ritual purposes, we do not extend Tashlumim or refrain from Tachanun through the 13th based on korbanot. However, regarding Isru Chag, since it follows directly after Yom Tov even outside Eretz Yisrael, its observance remains relevant—such as refraining from fasting even on Ches Sivan (the eighth), which would be Isru Chag following two days of Yom Tov.8. Summary: Practical Differences Between Eretz Yisrael and Chutz La’aretz
In summary: although in Eretz Yisrael Isru Chag and seven days of Tashlumim are directly tied to korbanot and thus end with Yud Beis Sivan (the twelfth), in Chutz La’aretz while we observe two festival days due to tradition (Sfeika deYoma) and maintain Isru Chag accordingly, we do not extend other aspects such as refraining from Tachanun or considering additional make-up days beyond what applies halachically for korbanot. This distinction continues to be analyzed further among poskim regarding its practical ramifications today.