"Why is it," Rebbe Levi Yitzchok of Berditchev was asked, "that Shavuot is also referred to as atzeret? After all, the Torah uses the term atzeret only in association with the festival of Shemini Atzeret, not Shavuot."
In reply, Rabbi Levi Yitzchok offered three explanations. He taught that all festivals have specific and positive 
Our experience of joy is defined not only by a feeling of past accomplishments, but also by forward-looking anxiety.
mitzvot associated with their celebration; that is, mitzvot in addition to the prohibition of work which is shared on all Yamim Tovim. For example, on Pesach we are commanded to eat matzah and drink four cups of wine; on Sukkot, we dwell in the sukkah and use the daled minim. But on Shavuot, we are commanded only to cease from our work. For this reason, Shavuot is known as atzeret; to signify its form of Yom Tov.
The Kedushat Levi went on to explain that the names of all Yamim Tovim identify a specific historical event that came to be, in subsequent generations, religiously commemorated. However, the name Shavuot does not identify any such event. Rather, it identifies the completion of the mitzvah of sefirat ha'omer, counting the omer. This raises an important question: Why is there a Yom Tov to signify the completion of an action? Wouldn't it be more sensible to celebrate and rejoice in anticipation of fulfilling a mitzvah, rather than in its completion? After all, we recite the she'hecheyanu prior to observing a mitzvah for exactly this reason - to thank God for allowing us to be able to observe a mitzvah which we haven't had the opportunity to experience for an extended period of time.
Isn't the feeling of anticipation much greater than the satisfaction of completion?
Judaism teaches that joy and religious ecstasy emanate from accomplishment and fulfillment. For the religious Jew, there is no greater joy than the celebration of a siyyum, the sharing and completion of a significant part of Torah. That said, our experience of joy is defined not only by a feeling of past accomplishments, but also by forward-looking anxiety. To have completed something is to confront finality, the end of things. It is for this reason that ardent and committed students of Torah proclaim hadran alach - 'I shall return to you.' For them, endings are also pivot points for new beginnings.
A genuine Jew celebrates not only the joys of yesterday, but at the same time, expresses hope for tomorrow. Yesterday without the hopeful tomorrow is too frightening to contemplate. In celebrating the past with an eye to the future, we pivot in time, lingering just long enough in yesterday to celebrate its joys, but not so long as to lose our hopes for the morrow.
Ultimately, this "lingering" is the essence of our Shavuot-Atzeret experience. Rashi teaches that our intrinsic need to linger is fundamental to Shemini Atzeret. We need to hold on to our joys and not allow them to come to an abrupt ending - shekashe alai pe'ridatchem. Shavuot marks the completion of weeks, of our counting of the omer. Atzeret looks forward, motivating us to continue to embrace the goals towards which we were counting.
Finally, the Kedushat Levi teaches that when a Jew experiences a religious awakening and reaches spiritual heights, there is a natural urge to transform the love, ecstasy and yearning into practical applications. Ramban interprets a verse in the Song of Songs - mah tairu u'mah te'oreru et ha'avah ad sh'techpatz - to mean that free and unfettered love must find a mode of practical expression.
We Jews are, ultimately, a practical people, a people of doing things in the world. Our religious experience does not leave us in contemplation on mountaintops, but in the hustle and bustle of the world, performing tikkun haolam. We hold that one should not, indeed cannot, love or worship God theoretically. Religious inspiration and faith demand ad sh'techpatz; it calls for a creation of a chefetz, a keli - an "instrument" through which to express and manifest our innermost feelings and understandings.
Yet, when the Jews at Sinai reached the highest levels of religious exultation and fervor they did not posses any practical means of expression other than the fulfillment of the command to hold back and refrain from "touching the mountain." They only had a negative precept to perform, a not-doing. The Yom Tov, then, is known as atzeret to recall the one and only commandment available to our ancestors who stood physically at Sinai, to recall the only "vessel" available to them to translate their deep and exulted religious feelings.
In these three explanations, Rebbe Levi Yitzchok also provides teachings that can serve as three approaches to the recitation and concept of Yizkor.
There are those who view the Jewish past as "ancient" history, with few lessons or consequences for the present or the future. For them, the past is "complete". It is gone. It does not "linger". For them, the spirit of Judaism is a lost in ancient winds, lacking any relevance or contemporary mode of expression. Their Jewish past is over and 
Rebbe Levi Yitzchok also provides teachings that can serve as three approaches to the recitation and concept of Yizkor.
done, having ceased with the passing of parents and grandparents. For them, the completion of the past holds no sway on the present, nor promise or hope for the future.
Then there are those Jews who find a more meaningful, if incomplete, method of remembering the past, of reciting Yizkor. They can recall a past that still "lives" in warm and tender feelings and sentiments. But it remains in the past. They refuse to allow the past - no matter how warmly recalled - to impact their present or future. These Jews respect the past, may even study and analyze it; they may research and publish studies about the world of our fathers and shtetl life - the world of yesterday. These are the Jews who recite Yizkor recalling the siyyum, but who never utter hadran alach - 'I shall return to you.'
For them, the past is sealed. It exists without the anxieties of kasha alai peridatchem - without the inspiration to carry on the legacy of those who taught us to count.
The only genuine method of reciting Yizkor compels us to do more than simply remember. It empowers us to be able to translate our memories, emotions and love of the past into new realities. Solomon exclaimed, "Why awaken or rouse the love, unless you are willing to create a new vessel to contain it?" Recalling the past has meaning only when one is able to translate the ahavah into a new chefetz.
The hard tears we cry as we remember tell us clearly that our present is only possible because of the past, and that any future must likewise be connected with the present. Remembering those who came before us, with their love and devotion, must simultaneously include an acknowledgement that their past is not only our present, but also the future for the next generation.
The past is complete. The future, our hope. We only linger in the present as we balance between the two.