Shavuot
The Festival of the Giving of the Torah — fifty days after Pesach, the early summer pilgrimage festival.
Summary. Shavuot (literally "weeks") falls on the 6th of Sivan in Israel (6th–7th in the diaspora). The Torah commands it as the conclusion of the seven-week count from Pesach (Vayikra 23:15–22). In rabbinic tradition, Shavuot is zeman matan torateinu — "the season of the giving of our Torah" — commemorating the revelation at Sinai. The customs include the all-night Tikkun Leil Shavuot (study session), the reading of the Book of Ruth, the eating of dairy foods, and the decoration of synagogues with greenery. The Ten Commandments are read in synagogue from a Torah scroll.
Tikkun Leil Shavuot
The all-night Shavuot study session is a Kabbalistic practice that originated with the 16th-century Safed mystics around the Arizal and Rabbi Yosef Karo. The custom is to stay up all night studying Torah, in symbolic preparation for receiving the Torah at Sinai (drawing on a Midrash that the Israelites overslept on the morning of the revelation, requiring God to wake them; the all-night study is the rectification). Standard Tikkun texts (printed in the Tikkun Leil Shavuot booklet) include passages from each of the books of Tanakh and from the Mishnah and Zohar. Contemporary practice often substitutes a series of shiurim on diverse topics.
The Ten Commandments and the Book of Ruth
On Shavuot morning, the Torah reading is the Ten Commandments (Shemot 19:1–20:23). The congregation traditionally stands for this reading. The Book of Ruth is read on Shavuot — five chapters about the Moabite convert who becomes the great-grandmother of David. Multiple reasons are given: Ruth's story takes place at the wheat harvest (which Shavuot also marks); Ruth as a convert parallels the entire Jewish people receiving the Torah at Sinai (the whole people as converts to the covenant); the tradition that David was born and died on Shavuot.
Dairy Foods
Many traditions hold that on Shavuot one eats dairy foods (cheesecake, blintzes, lasagna). Multiple explanations: (1) the Torah is likened to milk (Shir HaShirim 4:11); (2) on receiving the Torah and its kashrut laws, the Israelites did not yet have kosher meat ready, so ate dairy; (3) the gematria of chalav (milk) is 40, the number of days Moshe spent on Sinai. Whatever the reason, cheesecake is widely eaten.
Bikkurim
In the Beit HaMikdash, Shavuot was the season for bringing the bikkurim — the first fruits of the seven species (wheat, barley, grape, fig, pomegranate, olive, date) — in a public ceremony described in the Mishnah (Bikkurim 3). The Torah passage of mikra bikkurim ("my father was a wandering Aramean" — Devarim 26) was recited. Contemporary Israeli secular kibbutzim revived a folk-festival form of bikkurim. The full mitzvah awaits the rebuilding of the Temple.
Where Denominations Diverge
Universal observance. The Reform movement substantially recovered Shavuot in the 19th century with the institution of Confirmation, a ceremony for teenagers (16–18) marking the completion of religious education, held annually on Shavuot in many Reform congregations. Confirmation is now also widely practiced in Conservative and some Modern Orthodox settings. Tikkun Leil Shavuot has become near-universal across denominations.
Sources
Torah: Vayikra 23:15–22; Devarim 16:9–12; Bamidbar 28:26–31.
Megillat Ruth.
Mishnah, Bikkurim.
Talmud Bavli, Pesachim 68b — "on Shavuot all agree that it must also be for you (lachem)."
Rambam, Mishneh Torah, Hilchot Yom Tov.
Shulchan Aruch, Orach Chaim 494.
Further Reading
Yitz Greenberg, The Jewish Way — Shavuot chapter.
Rabbi Lord Jonathan Sacks, Covenant & Conversation: Numbers — Shavuot essays.