The dimension of the soul that corresponds to the Sefira of Kesser, is not the attributes through which we express our souls, but rather the very essence of the soul. The essence of the soul is referred to as the Yechidah, which means oneness. This reflects the indivisible oneness that the soul has with Hashem.
Based on the paradigm above, Shavuos is a one-day Festival because on this day, the essence of the soul is manifest. In this regards, Shavuos is like Rosh Hashana and Yom Kippur, which are also one-day Festivals, being days where the essence of the soul and our connection to Hashem are revealed.
This difference also explains why Pesach and Sukkos have specific, unique Mitzvos. Mitzvos represent an external connection to Hashem that is created through something that we do. Shavuos has no specific Mitzvos, because it represents the essential, all-encompassing bond of the essence of the soul with Hashem. This connection is not about doing something, it just is.
When we serve Hashem with only the conscious dimensions of the soul represented by the seven Sefiros, our service will be limited. The influence of this type of service is limited to that which is already conducive and naturally receptive to holiness. This is like the Korbanos of Pesach and Sukkos that could only be brought at a time which is also holy.
The power of the Yechidah which is revealed on Shavuos, is that when we are connected on this level, we can go into the most mundane parts of our lives and our world, and transform them to become holy.
The Frierdike Rebbe relates that the Tzemach Tzedek once asked the Alter Rebbe why Shavuos is celebrated for 7 days up until the 12th of Sivan. The Alter Rebbe answered with an analogy of a group of merchants who travelled to a fair that lasted for two days. At the end of the fair they would return home. But before departing, they would linger for a few days as they packaged and secured their merchandise, to ensure that nothing would be lost or damaged on the long journey home.
On Shavuos, we all experienced the revelation of Matan Torah. As we acquired the Torah, we experienced the revelation of Anochi - Hashem’s essence, and the revelation of our Yechidah.
We now need to secure our ‘merchandise’, to firmly internalise these revelations, so that they will last through our travels and trials in the year ahead.
