In our prayers we describe Shavuos as "the season of the Giving of our Torah." 
Our Sages teach,  however, that "Avraham fulfilled the entire Torah before it was given," that he communicated it to his descendants, and that even throughout their servitude in Egypt, the Jews studied the Torah. 
If so, what is meant by the Giving of the Torah?
This question can be resolved by reference to our Sages' statement  that, "a person who observes a mitzvah because he is commanded to do so is greater than one who observes it without having been so commanded."
Intellect is the most refined of our potentials.
And yet the finite mind of mortal man cannot leap across the chasm to G-d's infinity, for "no thought can grasp Him at all." 
There need not be, however, a separation between them.
By giving man the Torah and its mitzvos, G-d has built a bridge that enables man to establish a bond with His Maker on His terms. The word mitzvah and the word tzavta meaning "together", share the same root. 
When a person fulfills a divine command because he has been commanded to do so, the act which he performs is G-dly, and connects him to G-d in all His infinity.
If he performs the same deed without having been commanded to do so, his act, however worthy, remains merely a good deed: it does not establish a connection of the same nature.
Before the Torah was given, the Patriarchs were able to perceive its spiritual truths through prophetic vision. Realizing the inherent value of a life governed by these truths, they structured their lives accordingly and observed even the minutest details of Torah law.
However, since the Torah had not yet been given by G-d, and they had not been commanded to accept it, their observance did not lead to the same spiritual bond which is effected by the Jews' observance of the commandments after the Giving of the Torah.
Not only in the spiritual realms, but in the material world, too, a deed performed as a mitzvah and a deed performed without the obligation of a mitzvah produce different effects.
For example, the Zohar  states that when Yaakov laid out the staves of poplar, almond and chestnut trees before Lavan's flocks,  his actions evoked the same spiritual energies as are drawn down to this world through our performance of the mitzvah of tefillin. After the spiritual service associated with these staves was completed, however, they remained ordinary pieces of wood.
The spiritual influences they evoked left no lasting effects upon them. In contrast, when a Jew puts on tefillin, the tefillin become sacred objects; their holiness affects their very physical being. 
What is the reason for this difference?
Since Yaakov's deed was not an obligatory mitzvah, it lacked the infinite power that flows from G-d's essence. For this reason, the influence of his actions remained on the spiritual plane and did not affect the physical nature of the staves.
The mitzvos we perform, by contrast, connect us to G-d in an infinite bond; they are therefore able to infuse spirituality even into the material substance of this world.
This difference is reflected in our Sages' Midrashic comment:  "All the mitzvos which the Patriarchs fulfilled before You were vaporous, whereas to us may be applied the phrase,  'Your name is like flowing oil.'"
The mitzvos fulfilled by the Patriarchs were ethereal, whereas the mitzvos we fulfill manifest a tangible outpouring of the Divine Presence into our material world. 
Joining the Physical and the Spiritual
The above contrast is mirrored in another teaching of the Midrash: 
David said: "The Holy One, blessed be He, decreed,  'The heavens are the heavens of G-d, and the earth He gave to men....' Nevertheless, when He desired to give the Torah, He nullified that original decree and said, 'The lower realms shall ascend to the higher realms and the higher realms shall descend to the lower. And I shall take the initiative.' As it is written,  'And G-d descended on Mount Sinai,' and 'To Moshe, He said Ascend to G-d.'" 
In other words: According to the original pattern of creation, the material and the spiritual were confined to separate realms of existence, to discrete planes that never converge. The ultimate divine intent, however, was to fuse the two, so that the underlying G-dliness would surface within our material world.
Because G-d's essence is truly unlimited, this is possible: the spiritual can descend and become manifest within our world, and our worldly experience can be elevated beyond material concerns and become an expression of spiritual truth.
"No Longer in the Heavens"
This fusion cannot be accomplished through human enterprise alone: it is possible only because (as G-d says) "I shall take the initiative." This is what is unique about the Giving of the Torah at Mount Sinai. The divine revelations before Sinai had not been not intended to resolve the fundamental conflict between the spiritual and the material; the revelation at Sinai, by contrast, was intended to permeate the totality of existence.
And this it did.
"No bird chirped, no fowl took flight..."  - the entire natural order came to a standstill. Moreover, "His voice did not have an echo." 
Instead of rebounding, G-d's voice permeated the material substance of the world. From that moment onward, "The Torah is not in the heavens,"  but rather part and parcel of the environment in which we live.
So that the World Itself Should See
This was, however, merely the beginning of a process.
The ultimate purpose of eliminating the gap between spirituality and material existence was the second phase, "And Moshe ascended to G-d," i.e., that man should elevate himself and the material environment in which he lives.
As long as the connection between man, the world, and G-d is dependent on G-d alone, the fusion between these elements is incomplete. If G-dliness permeates the world only as a result of a revelation from above, the world remains - at least from its own earthbound perspective - separate from G-d.
This may be understood by comparing the world to a student who is able to arrive at a concept only when nurtured by his teacher's explanations. Only when he has reached the point at which he can conceive of the idea on his own, can we say that his thought processes have fully matured.
The service of G-d epitomized in the phrase, "And Moshe ascended to G-d," demonstrates just such a process of maturation within man and within the world at large.
Man's divine service refines the world and transforms it into a vessel for G-dliness, e nabl ing the world and its inhabitants to perceive G-dliness not as an externally supplied factor, but rather as the truth of its own existence. 
The consummation of our efforts to refine the world will come in the Era of the Redemption when we will merit the fulfillment of the prophecy,  "And all flesh will together see that the mouth of G-d has spoken" - not that the revelation from above will be so intense as to reach down to our material realm, but that material flesh will have an independent appreciation of G-dliness.
That era is fast approaching.
"All the spiritual tasks G-d has demanded of the Jewish people have been completed.... All that is necessary now is for each of us to open his eyes."  And then we will behold the ultimate purpose of the Giving of the Torah - the manifestation of G-d's presence throughout the world.
May this take place speedily in our days.
Adapted from Likkutei Sichos, Vol. III, Parshas Lech Lecha; Vol. VIII, Shavuos
1. Unlike the other festivals, whose calendric dates are specified in the Torah, Shavuos is not necessarily celebrated on the sixth of Sivan, the anniversary of the Giving of the Torah; it is celebrated on the fiftieth day after the beginning of the Counting of the Omer.
Thus, before the institution of a fixed calendar, when the first day of each Jewish month was determined by the testimony of witnesses who had seen the new moon, Shavuos could also be celebrated on the fifth of Sivan or on the seventh. (See Rosh HaShanah 6b; Shulchan Aruch HaRav 494:1; Likkutei Sichos, Vol. III, p. 997 ff.)
Nonetheless, although Pesach, the Counting of the Omer, and Shavuos represent three phases in a single pattern of divine service (see the above essay entitled "Sefiras HaOmer: Counting More Than Days"), many sources (e.g., Pesachim 68b; the Jerusalem Talmud, Rosh HaShanah 4:8) associate the celebration of Shavuos with the Giving of the Torah.
2. Kiddushin 82a; Yoma 28b.
3. Yoma, loc. cit.
4. Kiddushin 31a.
5. Introduction to Tikkunei Zohar 17a; see also Tanya, ch. 4.
6. See Likkutei Torah, Parshas Bechukosai 45c.
7. Vol. I, p. 162a.
8. See Bereishis 30:37.
9. This is why tefillin must be treated with respect even when they are not being worn. See Shulchan Aruch, Orach Chayim, sec 40.
10. Shir HaShirim Rabbah on Shir HaShirim 1:3.
11. Shir HaShirim 1:3.
12. The Glosses of R. Ze'ev Wolf Einhorn to Shir HaShirim Rabbah, loc. cit.
13. Shmos Rabbah 12:3.
14. Tehillim 115:16.
15. Shmos 19:20.
16. Ibid. 24:1.
17. Shmos Rabbah 29:9.
18. Ibid.; see also Likkutei Sichos, Vol. IV, p. 1092 ff.
19. Cf. Shabbos 89a.
20. Although this stage is the ultimate goal, it must be complemented by the stage in which "G-d descended...." This is because, as explained above, the initial step in bridging the chasm between spiritual and material existence must be made by G-d.
Moreover, man's divine service can relate only to those dimensions of G-dliness that are connected with the realm of finite existence. For the world to be connected with infinite G-dliness, there has to be a revelation from above.
21. Yeshayahu 40:5.
22. Sound the Great Shofar (Kehot, N.Y., 1992), pp. 139-140.