There are a number of branches of Judaism. The Orthodox are very strict in exactly how they practice what they believe to be the requirements of their faith. If you have heard about Jews who will not even turn on a light or use a car or an elevator on the weekly Sabbath, they were likely members of this branch of Judaism.
Those who are part of the Conservative branch of Judaism are slightly less rigid in their practice, but still are very careful about such things as observing the Kosher laws regarding food, and not working on the Sabbath.
Those who are part of Reformed Judaism recognize very few strict requirements for daily living, and if they follow such practices as avoiding non-Kosher foods such as pork and shellfish, they may view their actions as merely part of a cultural way of life rather than as binding commandments from God.
Material in this series of articles on Jewish customs refers in particular to either Orthodox or Conservative Jewish practices. For simplicity, the customs are described as they are scheduled in Israel--outside that country, every annual holy day as described in the Bible (except for the "fast day" of Yom Kippur) is traditionally observed for two days in a row. For an explanation on the reasoning behind this practice, see:
http://www.jewfaq.org/holiday0.htm
Leviticus 23:15-17, 21
From the day after the Sabbath [in the spring near Passover], the day you brought the sheaf of the wave offering, count off seven full weeks. Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to the LORD. From wherever you live, bring two loaves made of two-tenths of an ephah of fine flour, baked with yeast, as a wave offering of firstfruits to the LORD. ... On that same day you are to proclaim a sacred assembly and do no regular work. This is to be a lasting ordinance for the generations to come, wherever you live.
See the Scriptures for the Feast of Pentecost collection for more verses related to this Feast.
Shavuot in the 1st Century
During the Feast of Unleavened Bread (see the Pesach and Chag ha-Matzot article elsewhere on this Times of Refreshing website for details about the Jewish customs connected to that observance), the Jews of first century Israel would begin the fifty-day count to the next festival, Shavuot. (The Greek term for "counting fifty," also used in English, is Pentecost. See the Feast of Pentecost Scripture Collection for a concise set of the primary passages from the Old and New Testament related to the observance of this day.)
The Feast of Unleavened Bread, Shavuot (Hebrew word meaning "weeks," i.e., the Feast of Weeks because of the count of seven weeks), and the Feast of Tabernacles (Hebrew: Sukkkot) were the three "pilgrimage festivals" of the ancient sacred calendar as it was observed in the first century. All adult male Jews from all over the nation were required to make the trip to Jerusalem for the observance of these times, and they would usually bring their whole families if possible.
Shavuot
came at the end of the barley harvest that had begun at
Passover time, and marked the beginning of the summer
wheat harvest. It was thus also known as the "Feast of
Harvest." At the Temple on Shavuot, the High Priest
would present a symbolic offering to God of two loaves
of leavened bread made from the wheat of the new
harvest. (This is the only time leavening was used in
any ceremony at the Temple. Year around, only unleavened
bread was included as part of any sacrifice or
offering.)
Shavuot also came as some of the other agricultural products of the land were beginning to ripen. From all over Israel, farmers would bring the "firstfruits" (Hebrew: bikkurim) of their crops of what came to be known as the "seven species" to be presented before the Lord as an offering:
From the Wikipedia.com Shavuot article
These species are: wheat, barley, grapes, figs, pomegranates, olives, and dates (Deut. 8:8). In the largely agrarian society of ancient Israel, Jewish farmers would tie a ribbon around the first ripening fruits from each of these species in their fields. At the time of harvest, the fruits identified by the ribbon would be cut and placed into baskets woven of gold and silver. The baskets would then be placed on oxen whose horns were laced with garlands of flowers, and who were led in a grand procession to Jerusalem. As the farmer and his entourage passed through cities and towns, they would be accompanied by music and parades.
This was in fulfillment of a command found in Deuteronomy:
Deuteronomy 26:2
You shall take some first fruits of the various products of the soil which you harvest from the land which the Lord, your God, gives you, and putting them in a basket, you shall go to the place which the Lord, your God, chooses for the dwelling place of his name.
Although the command does not specify that this should be done at the time of Shavuot, by the first century that was the custom. For a plausible explanation why the first fruits were given only for the seven species, see
See the Feast of Pentecost Scripture Collection for a concise set of the primary passages from the Old and New Testament related to the observance of this day. There is nothing in these passages that indicates a special symbolic meaning for the observance of the day, other than that it is a time of thanksgiving for the harvest. But by the first century, its observance had also been connected to a commemoration of the giving of the Law by God to the nation of Israel at Mt. Sinai. The Biblical account of that event doesn't definitively clarify that it came exactly on the date of Shavuot, but tradition had long since assigned it to that date.
Shavuot in the 21st Century
But Shavuot has no such commandments connected to it, other than the ritual requirements that were in effect when a Temple was standing in Jerusalem and a priesthood was available to perform sacrifices and ceremonies. There are, however, customs and traditions that have grown up around the celebration of the day that are scrupulously observed by most observant Jews.
On
the evening that begins the day of Shavuot, many adults
stay up all night to study the Torah. This may be done
in the home with the family, although many synagogues
offer special classes and discussion sessions to fill
the night. Many Jews who live in--or are
visiting--Jerusalem traditionally finish their all-night
Torah study by walking to the Western Wall (Hebrew:
Kotel) at the Temple Mount and offering
prayers there at sunrise.
Shavuot sunrise at the Western Wall courtesy Bibleplaces.com

Another tradition that has developed in Judaism is the practice of reading the book of Ruth from the Bible at the synagogue on Shavuot. The explanation for this custom is usually that the events of the book of Ruth seem to primarily occur during the season of the barley and wheat harvests in Israel.
Other festive Shavuot traditions:

Homes and synagogues are
decorated with greenery, flowers (particularly roses and
lilies), plants, and fruit.

Jewish children often participate in craft projects to make floral wreaths for the girls to wear on their heads for Shavuot...
...and
baskets of fruit, flowers, and greenery that boys and
girls
will carry on their shoulders in Shavuot parades.

Foods made
from dairy products are particularly featured
on the menu for the day's feasting,
such as cheese blintzes and cheesecakes.

Gift and flower shops that cater to a Jewish clientele have large selections of items considered appropriate for Shavuot gift-giving, including fruit and cheese baskets, floral bouquets, and plants.

For one effective, detailed overview of the Jewish observance of the Feast of Pentecost, including explanations for the traditional customs, see the Shavuot article on the Wikipedia.com website. The material above was in part adapted from that overview.
http://en.wikipedia.org/wiki/Shavuot
Transliteration note:
The appearance of the Hebrew alphabet has nothing in common with the English alphabet. A number of Hebrew letters have sounds that are not distinctly connected to separate sounds in English. And even Hebrew itself is spoken in a wide variety of dialects depending on the cultural background of the speaker. Therefore attempts to transliterate (reproduce the sounds of a word or phrase in letters from one language to another) are always speculative at best. Even within the English-speaking community of Jews, there are wide variations in the way that Hebrew words are transliterated into English, with no central standard to draw from. In the articles in this series on Jewish Feast and Holy Day Customs, as well as throughout this Times of Refreshing website in general, in order to follow some sort of systematic way of transliterating, an attempt has been made to choose those transliterations that are most typical on the Internet. For instance, the Hebrew term for the Feast of Tabernacles is transliterated on some websites as Sukkot (1,160,000), others as Sukkos (119,000), others as Sukkoth (76,600), still others as Succot (128,000) or Succoth (162,000). In this case, the most typical choice is obvious, so Sukkot is the spelling of choice on this site. In other cases, the variations in spelling are much closer in numbers, and an arbitrary decision has had to be made.






