Introduction to Shavuot

Shavuot (Yom HaBikurim in the Torah) celebrates and commemorates the giving of the Ten Commandments to the Israelites at Mt. Sinai.
Beginning with the second night of Passover, the Israelites who left Egypt underwent 49 days of spiritual improvement and purification until they were ready to receive the Torah from God (Shavuot ends this 49-day “omer” period).  We can undergo a similar process of spiritual development each year during these 49 days (how to do that is beyond the scope of this website). According to our tradition, the Israelites in Egypt had sunk to the 49th level of spiritual impurity (tum'a). The Israelites had to raise themselves in 49 daily stages to be worthy of receiving the Torah.  Several books and siddurs portray the 49 days of the omer as corresponding to the Seven Sefirot embedded in the seven weeks.  This awareness can help us work on and maximize the power inherent in each day of the omer to fix that particular sefira in ourselves. We thus relive this transition from slavery to freedom and the service of God each year as we try to perfect our midot (personal characteristics) to again be worthy of receiving the Torah on Shavuot.

 
Symbolism of the Shavuot Offering
In the Temple in Jerusalem, the only communal sacrifice of leavened bread was on Shavuot. Leavening in dough is compared to arrogance in humans (people puff themselves up to look more important than they actually are). During Passover we destroy, and refrain from eating, leaven--just as we try to destroy/remove arrogance from our personalities. After Passover, we continue to work on our personal traits (midot) until we reach Shavuot, when we celebrate receiving the Torah.  At Shavuot, we Jews have a right to feel important, since we are spiritually elevated by virtue of having been given the Torah.
 
Shavuot: Universal Customs
The universal custom is to eat at least one dairy food during Shavuot.
Possible reason  At the time the Israelites received the Torah, they did not have any kosher meat (they had not been required to eat kosher until then) and so the only food they were permitted to eat was dairy food.
Another universal custom is to stay awake all night (if possible) studying Torah.
 
Shavuot: Symbols
Unlike other Jewish festivals, Shavuot has no concrete symbols and no specific unique commandments/mitzvot, other than sacrifices that were brought in the Temple in Jerusalem.