The sanctity of Shabbat is fixed and enduring. Since God created the world in six days and rested on the seventh, Shabbat is always on the seventh day of the week. In contrast, the sanctity of the festivals depends upon the Jewish people. This dependency is twofold. First, the unique idea of each festival was revealed through the Jews. On Pesaḥ, God redeemed the Jews from Egypt; on Shavu’ot, God gave the Torah to the Jews; and on Sukkot, we remember the special divine providence experienced by the Jews. On Rosh Ha-shana, the Jews stand as emissaries for all creation, crowning God as ruler of the world.
Second, in practice, the timing of the festivals depends upon the Hebrew calendar, whose months are sanctified by the Jewish people. In other words, even though a Hebrew month is based on the lunar cycle, seeing the new moon does not automatically inaugurate and sanctify the incoming month. Only the beit din can sanctify the month, based on Jewish attestations to seeing the new moon. The Torah instructs: “This month shall mark for you (lakhem) the beginning of the months” (Shemot 12:2). The Gemara expounds: “This testimony is handed over to you (lakhem)” (RH 22a).
It is true that we now have a set calendar instead. This is because approximately 300 years after the destruction of the Second Temple, the sages of Eretz Yisrael, under the leadership of Hillel the Second, understood that due to the exigencies of exile, it would be difficult for them to continue sanctifying the months. Therefore, they used a formula to calculate the calendar and to sanctify the months and years for the foreseeable future. Nevertheless, sanctification of the months is still dependent upon the Jews living in Eretz Yisrael, who calculate the months based on the formulas of the calendar and thus sanctify them. If, God forbid, the Jews were to disappear from Eretz Yisrael, the set calendar would not be binding, and the months and festivals would cease to exist. Fortunately, God promised us that this would never happen (MT, Laws of Sanctification of the New Moon 5:1-3; Sefer Ha-mitzvot §153; Peninei Halakha: Zemanim 1:3 n. 3).
We see that the sanctity of the festivals is dependent upon the Jews, which is why the Sages formulated the festival berakha in the Amida and Kiddush as “Blessed are You, Lord, Who sanctifies Israel and the seasons.” At first glance this would seem difficult. As is well known, we do not end a berakha by referring to two themes. Nevertheless, since the Jews sanctify the festivals, these two themes are not considered distinct; God sanctifies the festivals through the people of Israel (Berakhot 49a). In contrast, the sanctity of Shabbat is fixed and enduring, established by God. Accordingly, the formulation of the Shabbat berakha is “Blessed are You, Lord, Who sanctifies Shabbat” (Pesaḥim 117b). Therefore, even though Shabbat is holier than and superior to the festivals, there is more of a mitzva to rejoice on the festivals, because the value of our deeds in this world is more apparent then.
Since the sanctity of the festivals is dependent upon the Jews, the sanctity of the Jewish people is revealed on the festivals and is absorbed by each and every Jew. This expresses Jewish unity, as does each festival in its own way. On Shavu’ot, we received the Torah when we stood united facing the mountain (below 13:6). On Pesaḥ, the korban Pesaḥ hints at the unity of the Jewish people and its uniqueness (Maharal, Gevurot Hashem, pp. 36-37). On Sukkot, bundling together the four species expresses the unity among all parts of the nation.
Along these lines, in order not to create divisions among the pilgrims, on the festivals the Sages were lenient when it came to amei ha-aretz (those less knowledgeable). During the rest of the year, the Sages declared that the touch of an am ha-aretz rendered items impure, since some of them were not careful about the laws concerning purity and impurity. On the festivals, however, the Sages taught that one could rely upon their word for purity purposes. If an am ha-aretz declared that he was pure, he was to be believed, and his touch would not render food or sacrifices impure. The Sages connect this with the verse: “Gathered against the city were all the men of Israel, united as one man, friends” (Shoftim 20:11). We see that when everyone gathers together, they are all deemed friends, and thus reliable about matters of purity (Ḥagiga 26a; the Hebrew for friends is “ḥaverim,” which was also the rabbinic term for those who were careful about the laws of purity). The Sages also point to the verse: “Jerusalem is built like a city that is closely compacted together (ḥubra lah)” (Tehilim 122:3). We see that Jerusalem, the city of festival pilgrimage, turns all Jews into ḥaverim (y. Ḥagiga 3:6).