What is Hakhel?
The Torah explains that the people – all genders and all ages – would gather together every seven years to listen to the Torah being read publicly. This event, known as Hakhel, which means “gathering,” would take place during the week of the Festival of Sukkot and the leader of the people - usually the king - would recite specific passages.
Why is Hakhel during Sukkot?
Three times a year the Torah commands that people gather together - at Pesach in the early spring, at Shavuot in the early summer, and at Sukkot in the autumn - these are the three regalim, or pilgrimage festivals. The Torah instructs for Hakhel to happen during the festival of Sukkot:
(י) וַיְצַ֥ו מֹשֶׁ֖ה אוֹתָ֣ם לֵאמֹ֑ר מִקֵּ֣ץ ׀ שֶׁ֣בַע שָׁנִ֗ים בְּמֹעֵ֛ד שְׁנַ֥ת הַשְּׁמִטָּ֖ה בְּחַ֥ג הַסֻּכּֽוֹת׃ (יא) בְּב֣וֹא כׇל־יִשְׂרָאֵ֗ל לֵֽרָאוֹת֙ אֶת־פְּנֵי֙ יהוה אֱלֹהֶ֔יךָ בַּמָּק֖וֹם אֲשֶׁ֣ר יִבְחָ֑ר תִּקְרָ֞א אֶת־הַתּוֹרָ֥ה הַזֹּ֛את נֶ֥גֶד כׇּל־יִשְׂרָאֵ֖ל בְּאׇזְנֵיהֶֽם׃
(10) And Moses instructed them as follows: Every seventh year, the year set for remission, at the Feast of Booths, (11) when all Israel comes to appear before your God יהוה in the place that [God] will choose, you shall read this Torah aloud in the presence of all Israel.
What Happened at Hakhel?

The people would gather to have “this Torah” recited “in their ears.” As is often the case, there are different opinions among the commentators of what the Torah means when it says “this Torah.”

Does it literally mean that the entire Torah was read from the beginning to the end at once? Could it mean just the Book of Deuteronomy? Here’s what the rabbis in the Mishnah thought:
וְקוֹרֵא מִתְּחִילַּת אֵלֶּה הַדְּבָרִים עַד שְׁמַע וּשְׁמַע וְהָיָה אִם שָׁמֹעַ עַשֵּׂר תְּעַשֵּׂר כִּי תְכַלֶּה לַעְשֵׂר וּפָרָשַׁת הַמֶּלֶךְ וּבְרָכוֹת וּקְלָלוֹת עַד שֶׁגּוֹמֵר כׇּל הַפָּרָשָׁה
And the king reads from the beginning of Deuteronomy, from the verse that states: “And these are the words” (Deuteronomy 1:1), until the words: “Hear, O Israel” (Deuteronomy 6:4). And he then reads the sections beginning with: “Hear, O Israel” (Deuteronomy 6:4–9), “And it shall come to pass, if you shall hearken” (Deuteronomy 11:13–21), “You shall tithe” (Deuteronomy 14:22–29), “When you have made an end of the tithing” (Deuteronomy 26:12–15), and the passage concerning the appointment of a king (Deuteronomy 17:14–20), and the blessings and curses (Deuteronomy 28), until he finishes the entire portion.
According to this opinion, “this Torah” referred to a very specific collection of texts chosen from throughout the Book of Deuteronomy.

Even though in the time of the Torah there were no monarchs for the Jewish people, the rabbis of the Mishnah understood that it was the responsibility of the king, or, in their absence, the upper echelon of leaders who were expected to recite the Torah at Hakhel.

The entire procedure is described in the Mishnah:
חַזַּן הַכְּנֶסֶת נוֹטֵל סֵפֶר תּוֹרָה וְנוֹתְנָהּ לְרֹאשׁ הַכְּנֶסֶת וְרֹאשׁ הַכְּנֶסֶת נוֹתְנָהּ לַסְּגָן וְהַסְּגָן נוֹתְנָהּ לְכֹהֵן גָּדוֹל וְכֹהֵן גָּדוֹל נוֹתְנָהּ לַמֶּלֶךְ וְהַמֶּלֶךְ עוֹמֵד וּמְקַבֵּל וְקוֹרֵא יוֹשֵׁב אַגְרִיפַּס הַמֶּלֶךְ עָמַד וְקִבֵּל וְקָרָא עוֹמֵד וְשִׁבְּחוּהוּ חֲכָמִים וּכְשֶׁהִגִּיעַ לְלֹא תוּכַל לָתֵת עָלֶיךָ אִישׁ נׇכְרִי זָלְגוּ עֵינָיו דְּמָעוֹת אָמְרוּ לוֹ אַל תִּתְיָרֵא אַגְרִיפַּס אָחִינוּ אָתָּה אָחִינוּ אָתָּה ... בְּרָכוֹת שֶׁכֹּהֵן גָּדוֹל מְבָרֵךְ אוֹתָן הַמֶּלֶךְ מְבָרֵךְ אוֹתָן אֶלָּא שֶׁנּוֹתֵן שֶׁל רְגָלִים תַּחַת מְחִילַת הֶעָוֹן
The synagogue attendant takes a Torah scroll and gives it to the head of the synagogue that stands on the Temple Mount. And the head of the synagogue gives it to the deputy High Priest, and the deputy High Priest gives it to the High Priest, and the High priest gives it to the king. And the king stands, and receives the Torah scroll, and reads from it while sitting. King Agrippa arose, and received the Torah scroll, and read from it while standing, and the Sages praised him for this. And when Agrippa arrived at the verse in the portion read by the king that states: “You may not appoint a foreigner over you” (Deuteronomy 17:15), tears flowed from his eyes, because he was a descendant of the house of Herod and was not of Jewish origin. The entire nation said to him: Fear not, Agrippa. You are our brother, you are our brother...The same blessings that the High Priest recites on Yom Kippur, the king recites at this ceremony, but he delivers a blessing concerning the Festivals in place of the blessing concerning forgiveness for iniquity.
We can see that there was a public performance component to this ritual. The story about King Agrippa tells of a time when a foreign-born king showed humility and the Sages found his actions praiseworthy, also highlighting that this public event was truly for the entire Jewish people.

Why is Hakhel important?
The Torah does not specify *why* Hakhel happened. There are a few different reasons suggested by various voices over the years. One suggestion comes from the 13th century philosopher from Provence, Rabbi Levi ben Gershon, or the Ralbag, who teaches:
(א) ויצו משה אותם לאמר מקץ שבע שנים במועד שנת השמטה בחג הסוכות. צוה לקרא את התורה הזאת באזני העם האנשים והנשים והטף וגרי הצדק אשר עמהם כדי שיתחנכו כלם לעבודת הש"י ולא תשכח התורה מישראל ואמר כל הימים אשר אתם חיים על האדמה להורות על נצחיות התורה:
And Moses instructed them as follows: Every seventh year, the year set for remission, at the Feast of Booths - He commanded to recite "this Torah" in the ears of the people - men, women, children, and converts, in order to educate everyone to serve the Holy Blessed One and so the Torah is not forgotten from the Jewish people, saying that 'all of the days that you are on the land teach about the eternal nature of the Torah.'
For the Ralbag, Hakhel is important because it presents an opportunity every seven years for the people to be inspired by the Torah and its central place in Jewish culture.

To learn more about different understandings for what Hakhel is trying to accomplish, check out this sheet.
What Does Shemitah have to do with Hakhel?

This event is also designated to occur during Shemitah - the sabbatical year of agricultural and economic remission when every seven years farmland was left fallow and debts were released. The Shemitah year occurs every seven years beginning at Rosh Hashanah and ending at the following Rosh Hashanah. Since Sukkot takes place after Rosh Hashanah, the rabbis in the Mishnah interpret this to mean the Sukkot after the Shemitah year concludes:
מַתְנִי׳ פָּרָשַׁת הַמֶּלֶךְ כֵּיצַד מוֹצָאֵי יוֹם טוֹב הָרִאשׁוֹן שֶׁל חַג בַּשְּׁמִינִי בְּמוֹצָאֵי שְׁבִיעִית עוֹשִׂין לוֹ בִּימָה שֶׁל עֵץ בָּעֲזָרָה וְהוּא יוֹשֵׁב עָלֶיהָ שֶׁנֶּאֱמַר מִקֵּץ שֶׁבַע שָׁנִים בְּמֹעֵד וְגוֹ׳
MISHNA: How is the portion of the Torah that is read by the king recited at the assembly, when all the Jewish people would assemble? At the conclusion of the first day of the festival of Sukkot, on the eighth, after the conclusion of the Sabbatical Year, they make a wooden platform for the king in the Temple courtyard, and he sits on it, as it is stated: “At the end of every seven years, in the Festival of the Sabbatical Year” (Deuteronomy 31:10).
Were it not during or following a Shemitah year, the people would be busy preparing to plant grain to be harvested the following spring. However, having come out of a Shemitah year the people would not yet be engaged in such activities which allowed for more people to be present at Hakhel making it a true gathering of the people.