Counting the Omer
לִמְנ֣וֹת יָ֭מֵינוּ כֵּ֣ן הוֹדַ֑ע וְ֝נָבִ֗א לְבַ֣ב חָכְמָֽה׃
Teach us to count our days rightly, that we may obtain a wise heart.
וּסְפַרְתֶּ֤ם לָכֶם֙ מִמָּחֳרַ֣ת הַשַּׁבָּ֔ת מִיּוֹם֙ הֲבִ֣יאֲכֶ֔ם אֶת־עֹ֖מֶר הַתְּנוּפָ֑ה שֶׁ֥בַע שַׁבָּת֖וֹת תְּמִימֹ֥ת תִּהְיֶֽינָה׃ עַ֣ד מִֽמָּחֳרַ֤ת הַשַּׁבָּת֙ הַשְּׁבִיעִ֔ת תִּסְפְּר֖וּ חֲמִשִּׁ֣ים י֑וֹם וְהִקְרַבְתֶּ֛ם מִנְחָ֥ה חֲדָשָׁ֖ה לַיהוָֽה׃ מִמּוֹשְׁבֹ֨תֵיכֶ֜ם תָּבִ֣יאּוּ ׀ לֶ֣חֶם תְּנוּפָ֗ה שְׁ֚תַּיִם שְׁנֵ֣י עֶשְׂרֹנִ֔ים סֹ֣לֶת תִּהְיֶ֔ינָה חָמֵ֖ץ תֵּאָפֶ֑ינָה בִּכּוּרִ֖ים לַֽיהוָֽה׃

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: you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to God. 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 God.

Sefer HaChinuch, Mitzvah #306

Since the acceptance of the Torah was the goal of our redemption and serves as the foundation of the Jewish people, and through it we achieved our greatness, we were commanded to count from the day after [the first day of] Pesach until the day that the Torah was given. This manifests our great desire for that awesome day which our hearts yearn for just as a servant yearns for shade. We count constantly – when will the day come that we yearn for, the day that we left slavery? Because counting [towards a certain date] shows a person that all his desire and longing is to reach that time.

Rambam, Moreh Nevuchim (Guide for the Perplexed) 3:43

Shavuot is the time of the Giving of the Torah. In order to honor and elevate this day we count the days from the previous festival until it [arrives], like someone who is waiting for a loved one to arrive, who counts the days by the hours.

ReformJudaism.org

The counting of the Omer (S’firat HaOmer), which lasts for 49 days, takes place at night, commencing on the second night of Pesach. Those who "count the Omer" recite a blessing each night of the 50-day period:

בָּרוּךְ אַתָּה יְיָ אֱלֹהֵֽינוּ מֶֽלֶךְ הָעוֹלָם, אֲשֶׁר קִדְּשָֽׁנוּ בְּמִצְוֹתָיו, וְצִוָּֽנוּ עַל סְפִירַת הָעֹֽמֶר.

Baruch atah Adonai Eloheinu melech ha’olam asher kid’shanu b’mitzvotav vitzivanu al sefirat ha’omer.
Blessed are You, Adonai our God, Ruler of the universe, who sanctifies us with mitzvot, and commands us concerning the counting of the Omer.

The count is stated in both total days (“Today is the twenty-third day of the Omer,”) and weeks and days (“which is three weeks and two days of the Omer.”)