Shavuot is the anniversary of the Revelation on Mount Sinai, when God communicated the Torah to the entire Jewish people and in so doing, revealed to them their national mission and purpose. In our prayers, Shavuot is referred to as Zman Matan Torateinu, the time of the Giving of our Torah.
Unlike all the other holidays, no particular mitzvah is associated with Shavuot: On Passover, we eat matzah, on Sukkot we dwell in a sukkah and on Rosh Hashanah we blow the shofar, but no specific commandment is related to the giving of the Torah. Shavuot celebrates the totality of Torah. The Torah is all encompassing, it sanctifies and elevates every aspect of life; therefore it cannot be encapsulated in any single ritual, or any one experience.
On all festivals, the spiritual light that originally encompassed the world on the first occasion of that festival (for instance, the very first Passover) flows again every year at that same time. This increased spirituality is available to us if we are attuned to it.