Shavuot is a Jewish holiday celebrated either on the 6th or 7th of Sivan depending on that month's Rosh Chodesh, since it has to start when the Omer ends. It is the only Jewish holiday that is not celebrated on the same Jewish date every year. It is celebrated one day in Israel and 2 days outside of Israel. However, reform Jews celebrate one day outside of Israel as well.
Shavuot is celebrated because it is the day G-d gave Bnei Yisrael the Torah at Har Sinai. Since the Torah does not specify the actual day on which Shavuot falls, differing interpretations of this date have arisen both in traditional and non-traditional Jewish circles. These discussions center around two ways of looking at Shavuot: the day it actually occurs (i.e., the day the Torah was given on Mount Sinai), and the day it occurs in relation to the Counting of the Omer (being the 50th day from the first day of the Counting).
There are many traditions regarding Shavuot.
Eating dairy meals
Dairy foods such as cheesecake and blintzes with cheese and other fillings are traditionally served on Shavuot, mostly for the first meal. One explanation for the consumption of dairy foods on this holiday is that the Israelites had not yet received the Torah, with its laws of shechita (ritual slaughtering of animals). As the food they had prepared beforehand was not in accordance with these laws, they opted to eat simple dairy meals to honor the holiday. Therefore we follow their traditions today
According to the Midrash, Mount Sinai suddenly blossomed with flowers in anticipation of the giving of the Torah on its summit. Many people honor that by decorating their houses and shuls with plants and flowers in honor of Shavuot. Some synagogues decorate the bimah with a canopy of flowers and plants so that it resembles a chuppah, as Shavuot is mystically referred to as the day the matchmaker (Moses) brought the bride (the Jewish people) to the chuppah (Mount Sinai) to marry the bridegroom (God); the ketubbah (marriage contract) was the Torah. Some Eastern Sephardi communities actually read out a ketubbah between God and Israel as part of the service.
It is a custom to stay up the whole first night, and learn Torah the whole night. According to a story in the Midrash, the night before the Torah was given, the Israelites retired early to be well-rested for the momentous day ahead, but they overslept and Moses had to wake them up because God was already waiting on the mountaintop. To rectify this flaw in the national character, religious Jews stay up all night to learn Torah. Any subject may be studied, although Talmud, Mishnah and Torah typically top the list. In many communities, men and women attend classes and lectures until the early hours of the morning. In Jerusalem, thousands of people finish off the nighttime study session by walking to the Kotel before dawn and joining the sunrise minyan there.
Akdamut is a poem that speaks about the greatness of God. It is read by the Ba'al Koreh on the first day of Shavuot before the Torah reading. It was composed by Rabbi Meir of Worms, whose son was murdered during the Crusade of 1096. Rabbi Meir was forced to defend the Torah and his Jewish faith in a debate with local priests, and successfully conveyed his certainty of God's power, His love for the Jewish people, and the excellence of Torah. Afterwards he wrote Akdamut, a 90-line poem in Aramaic which stresses these themes. The poem is written in a double acrostic pattern according to the order of the Hebrew alphabet. In addition, each line ends with the syllable "ta" (תא), the last and first letters of the Hebrew alphabet, alluding to the endlessness of Torah. The traditional melody which accompanies this poem also conveys a sense of grandeur and triumph.
Sephardim do not read akdamut, but before the evening service they sing a poem called Azharot which sets out the 613 Biblical commandments. The positive commandments are recited on the first day and the negative commandments on the second day.
Book of Ruth
The Book of Ruth is read on the first day of Shavuot in Israel, and second day everywhere else. It corosponds to the holiday because of Ruth's desire to become a member of the Jewish people, who are defined by their acceptance of the Torah. Moreover, the lineage described at the end of the Book lists King David as Ruth's great-grandson. According to tradition, David was born and died on Shavuot