“And you shall take for yourselves on the first day….”1 With this verse, the Torah introduces the commandment to take the four species on Sukkos.וּלְקַחְתֶּם לָכֶם בַּיּוֹם הָרִאשׁוֹן וגו',
On this verse, the Midrash comments:2וְאִיתָא בַּמִּדְרָשׁ
“Is it the first day of the month? Clearly, it is the fifteenth [day]….3וְכִי רִאשׁוֹן הוּא וַהֲלֹא ט"ו [יוֹם] הוּא?
Instead, the intent is that [it]3 is the first day of the reckoning of sins.”אֶלָא רִאשׁוֹן [הוּא] לְחֶשְׁבּוֹן עֲוֹנוֹת.
For on Yom Kippur, the Holy One, blessed be He, forgives the Jews for all their sins.4דִּבְיוֹם הַכִּפֻּרִים הקב"ה מְוַתֵּר לְיִשְׂרָאֵל אֶת הַכֹּל,
From Yom Kippur until the festival of Sukkos,5 the entire Jewish people are busy preparing for mitzvos:6וּמִיּוֹם הַכִּפֻּרִים עַד הֶחָג כָּל יִשְׂרָאֵל עֲסוּקִין בְּמִצְוֹת,
one is occupied with building his sukkah, and yet another with preparing his lulav.זֶה עוֹסֵק בְּסֻכָּתוֹ וְזֶה בְּלוּלָבוֹ,
On the first day of the festival of Sukkos, the Holy One, blessed be He, says to them:וּבַיּוֹם הָרִאשׁוֹן שֶׁל חָג אוֹמֵר לָהֶם הקב"ה
“Whatever transpired previously, has passed. From this point onward, a new reckoning begins.”7מַאן דְּאָזַל אָזַל, וּמִן הָכָא וּלְהָלְאָה חֻשְׁבְּנָא.
From the above it is evident, that the observance of the mitzvah of taking the four species on the fifteenth of Tishrei is due to its being the first day of the reckoning.וּמִזֶּה מוּבָן, דְּמִצְוַת נְטִילַת ד' מִינִים בְּט"ו תִּשְׁרֵי הוּא מִפְּנֵי שֶׁהוּא רִאשׁוֹן לְחֶשְׁבּוֹן.
In particular, the association of the mitzvah of the four species with “the first day” is highlighted by a comparison to the mitzvos of sukkah and the festival sacrifices, which Scripture specifically associates with the fifteenth of the month of Tishrei. The fact that the mitzvah of taking the four species is not associated with that date, but rather with “the first day,” reflects a change of emphasis.
It is necessary to understand the connection between the first day of the new reckoning and the mitzvah of taking the four species.וְצָרִיךְ לְהָבִין הַשַּׁיָּכוּת דְּרִאשׁוֹן לְחֶשְׁבּוֹן לִנְטִילַת ד' מִינִים.
Moreover, explanation is required based on the statement of the Midrash:8וּבְיוֹתֵר צָרִיךְ בֵּאוּר עַל פִּי מַה דְּאִיתָא בַּמִּדְרָשׁ,
The Jews and the nations of the world come before the Holy One, blessed be He, on Rosh HaShanah, and face one another in judgment;יִשְׂרָאֵל וְאֻמּוֹת הָעוֹלָם בָּאִין לִפְנֵי הקב"ה בְּרֹאשׁ הַשָּׁנָה,
we do not know which one prevailed.וְלֵית אֲנָן יָדְעִין מַאן נָצַח.
When, however, the Jews take leave of the presence of the Holy One, blessed be He, with their lulavim and their esrogim in hand – triumphantly carrying them as symbols of victory –אֶלָּא בְּמַה שֶּׁיִּשְׂרָאֵל יוֹצְאִין מִלִּפְנֵי הקב"ה וְלוּלְבֵיהֶן וְאֶתְרוֹגֵיהֶן בְּיָדָן,
we know that the Jews prevailed.אָנוּ יוֹדְעִים דְּיִשְׂרָאֵל אִינוּן נִצּוּחַיָּא,
Therefore, Moshe charged the Jewish people, telling them, “Take for yourselves…”1לְפִיכָךְ מֹשֶׁה מַזְהִיר לָהֶם לְיִשְׂרָאֵל וְאוֹמֵר לָהֶם, וּלְקַחְתֶּם לָכֶם כו'.
The implication is that the mitzvah of taking the four species on the holiday of Sukkos comes as an extension of the judgment of Rosh HaShanah.הַיְנוּ, דְּמִצְוַת נְטִילַת ד' מִינִים בְּחַג הַסֻּכּוֹת בָּאָה בְּהֶמְשֵׁךְ לְהַדִּין דְּרֹאשׁ הַשָּׁנָה,
Why then does the verse say, “And you shall take for yourselves on the first day,”וְלָמָה אוֹמֵר וּלְקַחְתֶּם לָכֶם בַּיּוֹם הָרִאשׁוֹן,
emphasizing that taking the four species on this day isשֶׁמִּצְוַת נְטִילַת ד' מִינִים בְּיוֹם זֶה הוּא
(not due to its being the fifteenth day after Rosh HaShanah,(לֹא מִפְּנֵי שֶׁהוּא יוֹם הַט"ו לְרֹאשׁ הַשָּׁנָה,
but rather) because on this day something new begins.אֶלָּא) מִפְּנֵי שֶׁבּוֹ מַתְחִיל עִנְיָן חָדָשׁ,
It is the first day of the new reckoning.רִאשׁוֹן לְחֶשְׁבּוֹן.

Summary

The verse that introduces the commandment to take the four species states, “And you shall take for yourselves on the first day….” Instead of mentioning the date on which the mitzvah is to be performed – the fifteenth day of the month of Tishrei – as the Torah does regarding the mitzvos of sukkah and the festival offerings, it emphasizes that the mitzvah of the four species is to be fulfilled on “the first day,” underscoring that the first day of Sukkos begins a new motif. Even greater explanation is called for since the Midrash explicitly states that this mitzvah is fulfilled to demonstrate that the Jews prevailed in judgment on Rosh HaShanah.