The Sages called the festival of Shavuot: Atzeret [literally: restraint or cessation], because there are no specific mitzvot associated with the festival through which our connection to God is expressed. Therefore, it is a unique day, as the illumination that comes from above is not limited to specific types of connections.

There are two types of divine light: that which comes as a result of observing the mitzvot, and that which is tied to a particular time:

Why is Shavuot called Atzeret [in the Mishna], and likewise, why is the festival on the eighth day of Sukkot called Atzeret [see Leviticus 23:36]? There are two lights that the Creator shines upon the Jewish people. He shines the first one upon them because they fulfill His mitzvot. The other is not connected to the observance of mitzvot; rather, it comes as a result of the abundance of holiness at specific times. Now we will explain the difference between them: The light that comes as a result of the observance of mitzvot is limited and finite, as the mitzvot themselves are finite [that is, they each have a defined time, place, and procedure]. But the light that comes as a result of abundant holiness has had no limits set upon it.

When the divine light is revealed through a certain mitzva, the illumination corresponds to the mitzva and is therefore limited. But illumination that is not linked to a particular mitzva is unlimited:

On Sukkot and Passover the light shines due to the mitzvot that are fulfilled on these days, for example, the mitzvot of sukka and lulav on Sukkot and the mitzva of matza on Passover, in addition to the other mitzvot associated with these days. But on Shavuot and Shemini Atzeret, there are no unique mitzvot, and the light on these days has had no limits set upon it.

On these days of unlimited illumination, we must direct the light in accordance with our needs. The term Atzeret refers to the restraint and limitation of the light in furtherance of a certain purpose.

We must limit the light until the upper and lower worlds receive it, for if the light is unlimited, the lower worlds cannot hold it. Each individual must limit the light to the matter that he is in need of, for example, wisdom, children, life [health], or livelihood. For this reason these two festivals, that is, Shavuot and the eighth day of Sukkot, are called Atzeret. The word Atzeret hints to restraint and limitation, from the term: “And [the plague] was stopped [vate’atzar]” (Numbers 17:13). We must restrain and limit the light. (Rabbi Levi Yitzĥak of Berditchev, Kedushat Levi, Numbers, Shavuot)