(ט) וַיְדַבֵּ֥ר יהוה אֶל־מֹשֶׁ֥ה לֵּאמֹֽר׃ (י) דַּבֵּ֞ר אֶל־בְּנֵ֤י יִשְׂרָאֵל֙ וְאָמַרְתָּ֣ אֲלֵהֶ֔ם כִּֽי־תָבֹ֣אוּ אֶל־הָאָ֗רֶץ אֲשֶׁ֤ר אֲנִי֙ נֹתֵ֣ן לָכֶ֔ם וּקְצַרְתֶּ֖ם אֶת־קְצִירָ֑הּ וַהֲבֵאתֶ֥ם אֶת־עֹ֛מֶר רֵאשִׁ֥ית קְצִירְכֶ֖ם אֶל־הַכֹּהֵֽן׃ (יא) וְהֵנִ֧יף אֶת־הָעֹ֛מֶר לִפְנֵ֥י יהוה לִֽרְצֹנְכֶ֑ם מִֽמׇּחֳרַת֙ הַשַּׁבָּ֔ת יְנִיפֶ֖נּוּ הַכֹּהֵֽן׃ (יב) וַעֲשִׂיתֶ֕ם בְּי֥וֹם הֲנִֽיפְכֶ֖ם אֶת־הָעֹ֑מֶר כֶּ֣בֶשׂ תָּמִ֧ים בֶּן־שְׁנָת֛וֹ לְעֹלָ֖ה לַיהוה׃ (יג) וּמִנְחָתוֹ֩ שְׁנֵ֨י עֶשְׂרֹנִ֜ים סֹ֣לֶת בְּלוּלָ֥ה בַשֶּׁ֛מֶן אִשֶּׁ֥ה לַיהוה רֵ֣יחַ נִיחֹ֑חַ וְנִסְכֹּ֥ה יַ֖יִן רְבִיעִ֥ת הַהִֽין׃ (יד) וְלֶ֩חֶם֩ וְקָלִ֨י וְכַרְמֶ֜ל לֹ֣א תֹֽאכְל֗וּ עַד־עֶ֙צֶם֙ הַיּ֣וֹם הַזֶּ֔ה עַ֚ד הֲבִ֣יאֲכֶ֔ם אֶת־קׇרְבַּ֖ן אֱלֹהֵיכֶ֑ם חֻקַּ֤ת עוֹלָם֙ לְדֹרֹ֣תֵיכֶ֔ם בְּכֹ֖ל מֹשְׁבֹֽתֵיכֶֽם׃ {ס} (טו) וּסְפַרְתֶּ֤ם לָכֶם֙ מִמׇּחֳרַ֣ת הַשַּׁבָּ֔ת מִיּוֹם֙ הֲבִ֣יאֲכֶ֔ם אֶת־עֹ֖מֶר הַתְּנוּפָ֑ה שֶׁ֥בַע שַׁבָּת֖וֹת תְּמִימֹ֥ת תִּהְיֶֽינָה׃ (טז) עַ֣ד מִֽמׇּחֳרַ֤ת הַשַּׁבָּת֙ הַשְּׁבִיעִ֔ת תִּסְפְּר֖וּ חֲמִשִּׁ֣ים י֑וֹם וְהִקְרַבְתֶּ֛ם מִנְחָ֥ה חֲדָשָׁ֖ה לַיהוה׃ (יז) מִמּוֹשְׁבֹ֨תֵיכֶ֜ם תָּבִ֣יאּוּ ׀ לֶ֣חֶם תְּנוּפָ֗ה שְׁ֚תַּיִם שְׁנֵ֣י עֶשְׂרֹנִ֔ים סֹ֣לֶת תִּהְיֶ֔ינָה חָמֵ֖ץ תֵּאָפֶ֑ינָה בִּכּוּרִ֖ים לַֽיהוה׃ (יח) וְהִקְרַבְתֶּ֣ם עַל־הַלֶּ֗חֶם שִׁבְעַ֨ת כְּבָשִׂ֤ים תְּמִימִם֙ בְּנֵ֣י שָׁנָ֔ה וּפַ֧ר בֶּן־בָּקָ֛ר אֶחָ֖ד וְאֵילִ֣ם שְׁנָ֑יִם יִהְי֤וּ עֹלָה֙ לַֽיהוה וּמִנְחָתָם֙ וְנִסְכֵּיהֶ֔ם אִשֵּׁ֥ה רֵֽיחַ־נִיחֹ֖חַ לַיהוה׃ (יט) וַעֲשִׂיתֶ֛ם שְׂעִיר־עִזִּ֥ים אֶחָ֖ד לְחַטָּ֑את וּשְׁנֵ֧י כְבָשִׂ֛ים בְּנֵ֥י שָׁנָ֖ה לְזֶ֥בַח שְׁלָמִֽים׃ (כ) וְהֵנִ֣יף הַכֹּהֵ֣ן ׀ אֹתָ֡ם עַל֩ לֶ֨חֶם הַבִּכֻּרִ֤ים תְּנוּפָה֙ לִפְנֵ֣י יהוה עַל־שְׁנֵ֖י כְּבָשִׂ֑ים קֹ֛דֶשׁ יִהְי֥וּ לַיהוה לַכֹּהֵֽן׃ (כא) וּקְרָאתֶ֞ם בְּעֶ֣צֶם ׀ הַיּ֣וֹם הַזֶּ֗ה מִֽקְרָא־קֹ֙דֶשׁ֙ יִהְיֶ֣ה לָכֶ֔ם כׇּל־מְלֶ֥אכֶת עֲבֹדָ֖ה לֹ֣א תַעֲשׂ֑וּ חֻקַּ֥ת עוֹלָ֛ם בְּכׇל־מוֹשְׁבֹ֥תֵיכֶ֖ם לְדֹרֹֽתֵיכֶֽם׃
(9) יהוה spoke to Moses, saying: (10) Speak to the Israelite people and say to them: When you enter the land that I am giving to you and you reap its harvest, you shall bring the first sheaf of your harvest (OMER) to the priest. (11) He shall elevate the sheaf before יהוה for acceptance in your behalf; the priest shall elevate it on the day after the sabbath. (12) On the day that you elevate the sheaf, you shall offer as a burnt offering to יהוה a lamb of the first year without blemish. (13) The meal offering with it shall be two-tenths of a measure of choice flour with oil mixed in, an offering by fire of pleasing odor to יהוה; and the libation with it shall be of wine, a quarter of a hin. (14) Until that very day, until you have brought the offering of your God, you shall eat no bread or parched grain or fresh ears; it is a law for all time throughout the ages in all your settlements. (15) And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: (16) you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to יהוה. (17) You shall bring from your settlements two loaves of bread as an elevation offering; each shall be made of two-tenths of a measure of choice flour, baked after leavening, as first fruits to יהוה. (18) With the bread you shall present, as burnt offerings to יהוה, seven yearling lambs without blemish, one bull of the herd, and two rams, with their meal offerings and libations, an offering by fire of pleasing odor to יהוה. (19) You shall also offer one he-goat as a sin offering and two yearling lambs as a sacrifice of well-being. (20) The priest shall elevate these—the two lambs —together with the bread of first fruits as an elevation offering before יהוה; they shall be holy to יהוה, for the priest. (21) On that same day you shall hold a celebration; it shall be a sacred occasion for you; you shall not work at your occupations. This is a law for all time in all your settlements, throughout the ages.
We are commanded to Count the Omer in the seven-week period between Passover and Shavuot. Counting the days between these two holidays is a mitzvah and there are many interpretations for why we do so.
Torah: Agricultural
Passover and Shavuot are two of the three harvest festivals celebrated throughout the year. When the Temple stood, the people presented offerings on those two days and counted the days in between offerings, as mandated in the Torah. It is also the understanding that it should be the time where we thank God for what we have received.
Rabbinic: Spiritual
Rabbinic interpretation emphasizes the connection between Passover and Shavuot. Just as the Israelites moved in seven weeks from slavery in Egypt to freedom and Torah at Sinai, we count the Omer from Passover to Shavuot. The rabbis saw this as a way to express our thanks to God and in anticipation of the excitement of Shavuot, when we receive the Torah.
(א) עד ממחרת השבת השביעית תספרו. כבר ידעת שאין הספירה אלא מ"ט, ועל כן תמצא טעם במלת תספרו ואינה דבקה למלת חמשים, אבל חמשים דבק לוהקרבתם כי הספירה מ"ט והקרבן ביום חמשים, שלא הזכירה תורה חג שבועות ביחוד כשאר החגים ושיאמר כן בחדש השלישי בששי בו חג השבועות, כמו שאמר בפסח בחמשה עשר יום לחדש הזה חג המצות, בא ללמד כי הוא חג נגרר אחר מצות העומר, ומ"ט יום הספורים בין ראשון של פסח עד השבועות הנה הם כחולו של מועד בין ראשון של סוכות ושמיני עצרת.
....The Torah did not mention the Shavuot festival independently as it did with all the other festivals.... Seeing that the Torah did not refer to the festival Shavuot in that manner makes it plain that it is considered as a festival which is linked to the Passover, i.e. it completes what commenced on Passover and now that we have counted forty-nine days from the second day of Passover the time has come to celebrate the Shavuot festival. Looking at it from a spiritual point of view, the days between the last day of the Passover festival and the festival of Shavuot may be considered as somewhat like חול המועד, the intermediate days of the Passover and Sukkot festivals respectively.
Historical: A Period of Mourning
In the Middle Ages, the Omer began to be considered a period of semi-mourning when marriage and haircuts should be avoided in memory of the plague that struck down 24,000 of Rabbi Akiva's students. The plague ended on Lag Ba'Omer, the 33rd day of the Omer, which is celebrated today with bonfires.
ר"ע אומר למד תורה בילדותו ילמוד תורה בזקנותו היו לו תלמידים בילדותו יהיו לו תלמידים בזקנותו שנא' בבקר זרע את זרעך וגו' אמרו שנים עשר אלף זוגים תלמידים היו לו לרבי עקיבא מגבת עד אנטיפרס וכולן מתו בפרק אחד מפני שלא נהגו כבוד זה לזה
They said by way of example that Rabbi Akiva had twelve thousand pairs of students in an area of land..., and they all died in one period of time, because they did not treat each other with respect....With regard to the twelve thousand pairs of Rabbi Akiva’s students, the Gemara adds: It is taught that all of them died in the period from Passover until Shavuot. Rav Ḥama bar Abba said, and some say it was Rabbi Ḥiyya bar Avin: They all died a bad death. The Gemara inquires: What is it that is called a bad death? Rav Naḥman said: Diphtheria.
Talmud Yevamot, composed in Talmudic Babylon (c.450 - c.550 CE)
Also, "Lag Ba’Omer has evolved into a day when, in different Jewish communities around the world, the deaths of a number of tzaddikim (righteous men) are commemorated, in a festive fashion." (https://www.myjewishlearning.com/article/sefirat-ha-omer-time-as-text/)
Israelis: Independence
"For almost two millennia, from the mid-second century on, this is the way the Omer period was experienced, as a sad season, during which joyful activities were curtailed, punctuated by the minor festival of Lag Ba’Omer. Then, on May 5, 1948, David Ben-Gurion announced that the Jewish nation in Israel accepted the United Nations’ partition plan, and declared a state....Nineteen years later, during the Six Day War, when Israeli troops... liberated the Old City..., another holiday — Yom Yerushalayim — Jerusalem Day, was created." (https://www.myjewishlearning.com/article/sefirat-ha-omer-time-as-text/)
Counting the Omer is an Example of the Reality of Jewish Life
"...When it was realized that this same time period [of counting the Omer] also contains within it another dimension, another reality, that of the receiving of the Torah, the Rabbis did not hesitate to incorporate that into the way this period of time is experienced.... Crucially, however, the Jewish people also never erased anything. The more recent events which occurred during the Omer period, and our responses to them, were never allowed to supercede the older ones; they live, like commentaries and addendum on a page of Talmud, side by side, together, vying perhaps for our attention, but all given equal time.
This openness to the realities of our history, this willingness to notice and respond communally to events as they occur in the real world, and not only to see the world through the prism of pre-ordained understandings is, I believe, a particularly Jewish genius. The way we relate to time is multi-layered. Our past, our present, our future, are all here, with us. Nothing old is forgotten; nothing new is ignored. New events, sometimes contradictory ones, are assimilated into our personal and communal consciousness, as we try to balance our mourning of old tragedies with our celebration of new triumphs....
All of these events which occurred during the Sefirat Ha’omer period, along with everything else that we have gone through as a people, are remembered, commemorated, felt. They are, in fact, through our yearly experience of them, happening, again and again, in our memory and our imagination, as we continue to try to make sense of the unfolding text of Jewish history." (https://www.myjewishlearning.com/article/sefirat-ha-omer-time-as-text/)