Tikkun Leyl Shavuot
Staying up to study Jewish texts for all or part of the first night of Shavuot is an increasingly popular activity among a wide range of Jews. This custom is called a 
"tikkun." The word tikkun is most popularly understood to mean a process of "repairing the world" or even an improvement of one's qualities. Tikkun is also used to describe a particular time devoted to study, a seder--order--of texts designed for this study, and also a canonical book containing these texts .Excerpted from Celebrate! The Complete Jewish Holiday Handbook. Reprinted with permission from Jason Aronson Inc.
Following the holiday meal, many people proceed to synagogue for Ma'ariv [the evening service], followed by an all-night (or into-the-night, as many last only until midnight) Torah study session based on the kabbalists' [mystics'] practice. [This specifically refers to the sixteenth century mystics of Safed, Israel, under the leadership of Isaac Luria. Many people recite Ma'ariv before the meal, go home to eat, and return to synagogue for the study session. All in all, Tikkun Leyl Shavuot is a relatively recent development.]
We remain awake to show that, unlike the situation of our heavy-lidded ancestors at Sinai, there is no need to bring us to our senses; we are ready to receive Torah. The tikkun (which refers both to the study session and to the text used for it) was the only observance developed specifically for Shavuot. (Although tikkunim were later introduced for Pesach/Passover and Hoshanah Rabbah--which is part of Sukkot, the Festival of Booths--this is the one most widely observed.)
In addition to wanting to compensate for the Israelites, the mystics had the idea that at midnight the heavens open and favorably receive the thoughts, study, and prayers of those who remain awake on the anniversary of the Revelation. (Some promoted the legend that, as on Hoshanah Rabbah, wishes are fulfilled at that moment. The Moroccan Jews believed staying up all night guaranteed you life for the next year.) It was likened to the hours of preparation prior to a wedding. The chapter headings in the anthology used for study were said to represent the jewels used to adorn a bride prior to her marriage, and the tikkun process was the bedecken ceremony (when the groom verifies the identity of his bride and places the veil over her face), which precedes the wedding.
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Don't forget to celebrate Yom Yerushalayim on May 20th