Shavu’ot is not tied to a particular calendar date, but to a counting from Passover. Because the length of the months used to be variable, determined by observation as we have learned in the Jewish Calendar ,and there are two new moons between Passover and Shavu’ot, Shavu’ot could occur on the 5th or 6th of Sivan.
However, now that we have a mathematically determined calendar, and the months between Passover and Shavu’ot do not change length on the mathematical calendar, Shavu’ot is always on the 6th of Sivan.
It is customary to stay up the entire first night of Shavuot and study Torah, and then pray as early as possible in the morning.
It is customary to eat a dairy meal at least once during Shavuot. There are varying opinions as to why this is done. Some say it is a reminder of the promise regarding the land of Israel, a land flowing with “milk and honey.” According to another view, it is because our ancestors had just received the Torah and did not have both meat and dairy dishes available.
The book of Ruth is read at this time. Again, there are varying reasons given for this custom, and none seems to be definitive.
Since the Torah is called reishit (“first”), the customs of Shavuot highlight the importance of custom for the continuation and preservation of Jewish religious observance. These customs, largely observed in Ashkenaziccommunities, are:
אקדמות– Akdamut, the reading of a liturgical poem during Shavuot morning synagogue services
חלב– Chalav (milk), the consumption of dairy products like milk and cheese
רות– Ruth, the reading of the Book of Ruth at morning services
ירק– Yerek, the decoration of homes and synagogues with greenery
תוקה– Torah, engaging in all-night Torah study.