Choosing

In the Mishna the holiday of Shavuot is refered to as Atzeret.  Why does it bear this name if that is not the name the Torah says and if it is the correct name why does the Torah not call Shavuot Atzeret?  The holiday of Shavuot has no mitzvot attached to it as do the other holidays, why not?  The Magen Avraham (60:2) says when one says in ahavas olam/ahavah rabbah ובנו בחרת מכל עם ולשון one should have in mind Matan Torah.  What does Hashem's choosing of us have to do with Matan Torah?  

True bechirah is when chooses without any reason for the choice.  For the most part when one has a choice and chooses one way over the other, it is due to some reason or some favoritism to one choice over the other.  That is why one is hard pressed to find a choice made out of ultimate bechirah of purely choosing just for the sake of choosing.  When Hashem gave the Torah to Klal Yisrael is wasn't due to the positive traits within the nation but rather just due to the ultimate choice of Hashem.  Hashem desired, he chose Klal Yisrael to be the people to receive the Torah.  This is why Hashem offered the Torah to the gentiles.  Why bother if he knows they will not accept?  Because it shows that there was no favoritism in choosing Klal Yisrael, everyone had equal opportunity to obtain the Torah (Likutay Sichos volume 4 hasafos Shavuot.)  Reciprocally, Klal Yisrael chose to due the desire of Hashem.  Hence the aseres hadibros include basic commandments such as not to kill, steal, commit adultery etc. since even the most basic commandments are not fulfilled due to human understanding of the law but due to choosing to due the desire of Hashem (see Likutay Sichos volume 28 sicha of 12 Sivan.)  

The Kedushat Levi gives a few explanations of the name of the holiday being Atzeret.  One of his explanations is that there are no specific mitzvot attached to the holiday and the only thing that designates it as being a holiday is being עצור, stopping from work. In other words, every holiday has a mitzvah that reflects the mood of the holiday.  There is a reason, a logic for why we celebrate those days as special days.  On Shavuot, we are celebrating the ובנו בחרת, the innate choosing of Klal Yisrael by Hashem and that can not be limited to the container of specific mitzvot (see sicha ibid.)  However, why does the Torah not call the holiday Atzeret?  Possibly, the Torah refers to the holiday by what we have to do to be prepared, we have the weeks of preparation for Shavuot.  However, Klal Yisrael calls the holiday Atzeret to reflect the unlimited connection to Hashem established on the holiday.