What will happen is, when you enter the land which G‑d, your G‑d, is giving you as an inheritance, you should take possession of it and settle in it.
Then [when the land is fully settled], you should take some of the first [to ripen] from every fruit of the ground which you gather in, from your land that G‑d, your G‑d, is giving you.
You should put it into a basket and go to the place which G‑d, your G‑d, will choose to make His Name rest there.
-- Devarim 26:1-2
Classic Questions
When were first-fruits first brought? (v. 1-2)
Rashi: The words, "What will happen is, when you enter... you should take possession of it and settle in it," teach us that they were not obligated to bring first-fruits until they conquered the land and divided it.
Sifri: [The obligation to bring first-fruits] began immediately upon entering the land.
Malbim: How can Sifri argue that the obligation to bring first-fruits began immediately upon entering the Land of Israel, when the verse states explicitly that "you should take possession of it and settle in it" before bringing firstfruits? Rather, Sifri's intention here is that the mitzvah of first-fruits became obligatory immediately after conquering the land and dividing it. This is in contrast to the mitzvah of eliminating Amalek, recorded at the end of the previous parsha, which did not become obligatory immediately after conquering the land and dividing it, but only later, "when G‑d your G‑d gives you relief from all your surrounding enemies" (25:19).
The Rebbe's Teachings
When Did The Mitzvah of First-fruits Begin? (v. 1)
Rashi explains that the mitzvah of offering first-fruits did not become obligatory until the Jewish people had fully conquered and settled in the Land of Israel, a process which actually took fourteen years.
Sifri, however, is of the opinion that the obligation to bring first-fruits became obligatory immediately upon entering the Land of Israel.
One problem with the view of Sifri is that it appears to contradict that which is stated explicitly in verse 1, that the mitzvah of first-fruits only applies after the Jewish people "take possession" of the land "and settle in it"—as Malbim points out.
Malbim explains that Sifri's intention is not that the mitzvah became obligatory immediately upon entering the land, but rather, immediately upon settling in it. This is not indicated, however, in the wording of the Sifri.
The Explanation
The Torah stipulates that first-fruits are only to be brought after "you enter the land... take possession of it and settle in it" (v. 1). Rashi understands that, like the vast majority of mitvzos connected with entering the Land of Israel, this mitzvah would only become obligatory after the entire land was conquered and settled by the Jewish people as a whole.
Sifri, however, understands that the above verse is speaking to the individual; i.e., when you personally enter the land, you have an obligation to bring first-fruits immediately upon settling on your own private property. Thus, you do not have to wait until the entire land has been conquered and settled.
At first glance, however, this does not fully solve our problem. For even according to Sifri, a person only brings first-fruits upon acquiring his own piece of land (when he can speak of the land "which you, G‑d, have given to me," v. 10), and the allocation of land only began after the conquest was complete, seven years after the Jewish people entered the Land of Israel. Thus, even according to Sifri it appears that the obligation to bring first-fruits did not begin immediately, but rather after seven years.
It could be argued, though, that according to Sifri, it did in fact become obligatory for a person to bring first-fruits immediately upon entering the land, but until the person actually owned land, he was legally exempt from doing so. According to Rashi, however, the obligation to offer first-fruits did not commence until the Jewish people as a whole had entered, conquered and settled the entire land.
The Rationale of Rashi and Sifri
At first glance, Rashi's opinion also appears to be difficult to appreciate. Rashi himself writes that first-fruits are brought so as not to appear "ungrateful" to G‑d (v. 3), so surely when a person has fruits growing from his own land, he should make an offering to G‑d immediately, as Sifri argues? According to Rashi, a person who was one of the first to settle in the land would be enjoying fruits from his field for seven years before thanking G‑d!
In truth, however, there are different approaches to expressing our gratitude to G‑d, each having their own unique advantage. For example, according to Jewish custom, as soon as a person wakes in the morning he recites the prayer Modeh Ani as an expression of gratitude to G‑d, while he is still in bed, even before his hands have been washed. Later in the day, he will pray more at length to G‑d, at which time he will obviously be in a more fit state of mind to praise his Creator.
Nevertheless, each of these types of praise possesses a unique advantage: His recital of the Modeh Ani prayer in the morning is immediate and spontaneous, demonstrating a deep-rooted commitment to serving G‑d, to the extent that the very first thing a person does upon awakening is to recite a prayer. On the other hand, his praises to G‑d later in the day are more meaningful, because they arise from a fuller and more conscious appreciation of G‑d's kindness.
Sifri perceives the first-fruits to be a form of thanksgiving to G‑d of the former type (comparable to Modeh Ani), where a person offers fruits to G‑d immediately on acquiring his field.
Rashi, however, understands that the first-fruits are a form of praise which comes after a person fully appreciates the fact that G‑d gave the Land of Israel to the Jewish people (comparable to one's prayers later in the day that arise from a fuller appreciation of G‑d's kindness). So until the point has been reached where every Jewish person has received his portion in the land, one's praise to G‑d cannot be complete—for how can one Jew rejoice in receiving his portion when another Jew is still lacking? Thus, Rashi maintains that first-fruits are only brought after "they conquered the land and divided it."
When Rashi & Sifri Would Agree
It could be argued that the positions of Rashi and Sifri are not mutually exclusive. For while the conquest and division of the Land of Israel did actually take fourteen years, if the sin of the spies had not occurred, it would have happened instantaneously (for the spies themselves would have conquered the land—see Rashi on Bamidbar 21:32). Thus, in such an instance, first-fruits would have been brought immediately upon entering the land, even according to Rashi.
And this will indeed occur with the true and complete Redemption, when not only will the conquest of all the ten promised lands be instantaneous, but the fruits will grow instantly too, as the verse states, "the plowman will meet the reaper, and the trader of grapes the one who carries the seed" (Amos 9:13). Thus, at that time, both Rashi and Sifri will concur that first-fruits are to be brought immediately.
(Based on Likutei Sichos vol. 34, pp. 145ff.; vol. 9, pp. 155-6; Sichas Shabbos Parshas Ki Savo 5744; Sichas Shabbos Parshas Va'eschanan 5751)