The most obvious example of holidays that are separate but nevertheless maintain a relationship are Rosh Hashanah, the two-day Jewish New Year, and Yom Kippur, the Day of Atonement. We are used to speaking of these three days as "the High Holidays," but in fact there are technical reasons to consider the entire period from the first of Tishrei, when Rosh Hashanah beings, to the tenth of Tishrei, when Yom Kippur occurs, as a single period. What unites them is not merely the theological notion that (as the traditional prayer book has it) "on Rosh Hashanah [one's fate] is written down, and on Yom Kippur it is sealed." There is also a technical aspect that demonstrates the connection: the Ashkenazic liturgical custom of Yizkor, prayers in memory of the dead.
Yizkor prayers are recited on the eighth day of Passover (the seventh day in Israel) and the second day of Shavuot (in Israel, on the one and only day of the festival). But there is no Yizkor on Rosh Hashanah, because that festival is merely the beginning of a 10-day period that ends with Yom Kippur—and that is when Yizkor, at last, comes around. The festival of Sukkot, beginning on the 15th of Tishrei, likewise has no Yizkor, because it is followed immediately by a holiday that is marked as related to Sukkot by serving as its occasion for Yizkor: Sh'mini Atzeret.
That is the festival described in Leviticus 23:36, where Rashi's comment on the word Atzeret occurs, and this leads us back to the biblical year of festivals—not merely sparser than the religious calendar used by contemporary Jews, but organized into two quite distinct seasons.
The fall holidays culminate with Sukkot, the seven-day festival that begins on the full moon of the seventh month, the first month of fall. When Sukkot ends, the holidays stop for the year. Rashi's comment notwithstanding, the Atzeret the Israelites were commanded to observe on the eighth day (sh'mini is Hebrew for "eighth") did not commemorate their being asked by God to linger for just a few more hours. Rather, it marked the end, the stopping point, of the fall holiday season. What was expected to follow next, as it still is, were the six months during which rain falls in Israel.
At last spring comes, and on the full moon of the first month, which is also the beginning of spring, Passover arrives. Seven weeks later (hence "the Feast of Weeks"), that is, 49 days later (hence "Pentecost," marking the 50th day), Shavuot arrives—the only festival that does not have a calendar date. Instead, it is inextricably linked to Passover. In biblical times the link was agricultural (see Leviticus 23:15–21); nowadays, they are linked liturgically by the ritual known as the counting of the Omer.
And that is why the rabbinic Sages gave Shavuot the name Atzeret. As on "the Atzeret of the eighth day," today's Shemini Atzeret following Sukkot, Shavuot is more than just a holiday of its own. It marks the conclusion of the festivals of the spring season. With it, the first of the two original Jewish holiday seasons comes to a close. Who could blame the Israelites for wanting to be "detained," for just one more day, in the festive atmosphere of Jerusalem, before heading home for a summer of field labor? But they could not linger; pace Rashi, that is not why Shavuot was called Atzeret. Rather, it meant that the festivals of the beginning of the year were over, and summer—a season of hard work for the farmers envisioned by the book of Leviticus—was ready to begin.
Michael Carasik is the creator of The Commentators' Bible and of the Torah Talk podcast. He teaches at the University of Pennsylvania.