(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 לַה'׃
(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 ה'.
The Torah tells us to count 7 weeks from the day of the sheaf of grain offering (traditionally offered in the Temple on the second day of Pesach). This period of counting is called the Omer. The Omer ends with the holiday of Shavuot, when another offering would traditionally be brought.
אָמְרוּ: שְׁנֵים עָשָׂר אָלֶף זוּגִים תַּלְמִידִים הָיוּ לוֹ לְרַבִּי עֲקִיבָא מִגְּבָת עַד אַנְטִיפְרַס, וְכוּלָּן מֵתוּ בְּפֶרֶק אֶחָד, מִפְּנֵי שֶׁלֹּא נָהֲגוּ כָּבוֹד זֶה לָזֶה.
They said by way of example that Rabbi Akiva had twelve thousand pairs of students in an area of land that stretched from Gevat to Antipatris in Judea, and they all died in one period of time, because they did not treat each other with respect.
Rabbinic tradition teaches us that Rabbi Akiva lost 24,000 of his students to a plague during the Omer because they did not treat each other with respect. Because of this, the Omer is now observed as a period of semi-mourning. Many Jews traditionally refrain from haircuts, weddings, parties, live music, and more during this time.
נוהגים שלא להסתפר עד ל"ג לעומר שאומרים שאז פסקו מלמות
It is customary not to cut one's hair until Lag BaOmer, since it is said that that is when they stopped dying.
On the 33rd day (Lag BaOmer), it is taught that the plague ended and the deaths of Rabbi Akiva's students stopped. This day marks the lifting of mourning restrictions in many communities, and is a day of celebration and fun.
There is a wide range of observances for Lag BaOmer. It is commonly marked by building bonfires, singing, and dancing. Many also consider it an auspicious day for weddings.
According to Kabbalistic tradition, this day also is a celebration of Rabbi Shimon Bar Yochai, one of Rabbi Akiva's disciples. Some have a custom of traveling to Meron in Israel, where he is believed to be buried, for Lag BaOmer celebrations.

