The Shavuot festival lasts only one day. The following day is called Isru Chag—bound to the feast. This is the case in the Land of Israel. Outside of Israel, it is celebrated on that day as "the second day of the Diaspora feast." The reason why all the feasts are celebrated for an extra day stems from a question: According to biblical law, the months must be fixed and sanctified based on the eyewitness testimony of two people who saw the new moon. It is not based on a pre-established calendar, but rather was determined by the court—the Beit Din—of Jerusalem. Sometimes the first day of the month—Rosh Chodesh—was the thirtieth of the previous month, and other times the thirty-first. It all depended on when the witnesses who saw the new moon arrived. Of course, the day the month began had direct consequences on when the feasts of that month would be held. At that time, people outside of Israel did not learn in time when the new month was fixed, and therefore, they celebrated each feast for two days, due to the uncertainty of when the first day of the month was determined. month."DISPOSITION OF OUR WISE MEN
However, this reasoning does not apply to Shavuot, as there is no doubt about its date. The Torah does not assign Shavuot a specific date based on the days of the month, but rather mandates that it be celebrated 50 days after Passover. From Rosh Chodesh Nisan to Shavuot, there is ample time for even those living outside of Israel to know when Passover would fall, and of course, to have no doubt about the exact date of Shavuot.
The reason is that it is a ruling of our Sages. That is, even though the original reason for the second day of the Diaspora festival does not apply to Shavuot, they nevertheless established that it be celebrated on the second day, so as not to create differences between the festivals.
THE STRENGTH THAT THE TORAH GAVE
This elevates Shavuot to a higher level of certainty, in contrast to the doubt surrounding the other festivals. In the other festivals, the sanctity of the second day is uncertain, yet the sanctity of Shavuot is evident, and yet our Sages sanctified the second day and made it a festival. This festival also demonstrates the power the Torah bestowed upon the Sages of Israel to establish provisions even regarding biblical festivals, to the point that "the second day of the Diaspora festival" has a sanctity equivalent to a Torah festival, since fulfilling a rabbinic law is also a Torah commandment, as it is written: "You shall not deviate from what they tell you" (Deuteronomy 17:11. See Maimonides at the beginning of Hilchot Mamrim).
ABSOLUTE CERTAINTY
It is known that the Hebrew word for doubt is "safek," which has the same numerical value (240) as the word "Amalek." Just as the people of Amalek came to dampen the enthusiasm of the Jews on their journey to receive the Torah, so too does doubt dampen the fervor and trust of the Jew in the Most High. The second day of Shavuot, on which there is no doubt, motivates us to eliminate doubt and serve Hashem with the firmness of certainty.
It is in this spirit that the directives of the sages of Israel, the Tzadik and spiritual leader of the generation, must be fulfilled: Every commandment, whether from the Torah or rabbinic or even the embellishment of a mitzvah (hidur), all of this is included in the commandment of "based on the Torah that they will teach you," and therefore they must be fulfilled with total perfection, just as one fulfills an order of the Torah.
(Torat Menachem, Itvaadut 5710, Volume 1 Page 90)