Rabbi Yehudah said to [Rabbi Abba]: There is something I would like to know. We see that Pesach is [observed for] seven days, and Sukkot is also [observed for] seven days and the completion of the celebration is on the last day [the Seventh of Pesach and Shemini Atzeret, respectively]. Why isn't Shavuot [also celebrated for] seven days? Indeed, it would be even more appropriate [to celebrate Shavuot for seven days] than either of the other [festivals], ecause Shavuot is when the Torah was given to us.

Shavuot…is the reliable bond that joins all levels together….

Rabbi Abba began his discourse, saying: "Who is like Your nation, like Israel, a single nation on earth?". (Samuel II 7:23) Why is it that Israel is called "one nation" here rather than anywhere else? However, since [the Prophet] intends to express Israel's praise, he refers to them as "one". In fact, this is always the praise of Israel, that they are one nation. Why? Because the bond between the upper [sefirot - chesed and gevura] and the lower [sefirot - nezach, hod, yesod] is always in that place [tiferet], which is called "Israel". [Also] it binds those that are above [the sefirot of Zeir Anpin ] to those that are below, to the Community of Israel [Malchut]. Accordingly, [when they are all bound together] they are called "one". [The reason that tiferet alone is called "one" is because] in that place perfect faith [in G‑d] is made known [to the world]. It is also the perfect bond [uniting the upper and lower sefirot of Zeir Anpin, as well as the consummation of] the higher unity [binding Zeir Anpin to Malchut]. (Ramak; Mikdash Melech)

That is why this day [of Shavuot, which corresponds to tiferet] is the reliable bond that joins all [levels] together. And so it is written, "It is the Tree of Life for those who hold fast to it." (Proverbs 3:18)

This is because it sends out life to all the branches, like the trunk of a tree transmits sap from the roots to the branches and leaves.

Shavuot is not celebrated for seven days since it is entirely an expression of unity….

The tree is therefore referred to in the singular [the Tree of Life, not in the plural] since all levels are bound to it.

Shavuot is not celebrated for seven days since it is entirely an expression of unity.

So its day [its manifestation in time] is certainly only one, and it binds all and is the central point of all.

Shavuot corresponds to the sefira of tiferet, which is the central point and includes all the other sefirot of the partzuf of Zeir Anpin and malchut (Ramak; Mikdash Melech), whereas Pesach corresponds only to the sefira of chesed, and Sukkot only to the sefira of gevura - so they should be only one day.

Accordingly, it is written, "The Tree of Life was in the midst of the Garden [of Eden]" (Gen. 2:9) [in the midst of the sefirot surrounding it]. "In the midst" therefore means in the center, bound to all the sides [the sefirot that surround it and are dependent upon it] that are connected to it.

Because it [Shavuot= tiferet] is in the middle between Pesach [chesed] and Sukkot [gevura], the glory of the Torah [the Written Torah, corresponding to tiferet] is manifested on that day alone, and no more [i.e. not over several days, whereas Pesach and Sukkot, since they are the branches, are celebrated for seven days]. It is the glory of faith and binds everything together. (Ramak)

Rabbi Yehudah said: Blessed is the Merciful One, that I asked this question and merited these words!

[Zohar I:96a; translation and commentary by Moshe Miller]