Counting of the Omer (or Sefirat Ha'omer, Hebrew: ספירת העומר) is a verbal counting of each of the forty-nine days between the Jewish holidays of Passover and Shavuot. This mitzvah derives from the Torah commandment to count forty-nine days beginning from the day on which the Omer, a sacrifice containing an omer-measure of barley, was offered in the Temple in Jerusalem, up until the day before an offering of wheat was brought to the Temple on Shavuot. The Counting of the Omer begins on the second day of Passover (the 16th of Nisan) for Rabbinic & MESSIANIC Jews, and after the weekly Shabbat during Passover for Karaite Jews, and ends the day before the holiday of Shavuot, the 'fiftieth day.'.
Leviticus 23: Pesach and Feast of Matzot
4 “These are the appointed feasts of Adonai, holy convocations which you are to proclaim in their appointed season. 5 During the first month, on the fourteenth day of the month in the evening, is Adonai’s Passover. 6 On the fifteenth day of the same month is the Feast of Matzot to Adonai. For seven days you are to eat matzah. 7 On the first day you are to have a holy convocation and you should do no regular work. 8 Instead you are to present an offering made by fire to Adonai for seven days. On the seventh day is a holy convocation, when you are to do no regular work.”
Bikkurim and Shavuot
9 Adonai spoke to Moses saying: 10 “Speak to Bnei-Yisrael and tell them: When you have come into the land which I give to you, and reap its harvest, then you are to bring the omer[a] of the firstfruits of your harvest to the kohen. 11 He is to wave the omer before Adonai, to be accepted for you. On the morrow after the Shabbat, the kohen is to wave it. 12 On the day when you wave the omer you are to offer a male lamb without blemish, one year old, as a burnt offering to Adonai. 13 The grain offering with it should be two tenths of an ephah of fine flour mixed with oil—an offering made by fire to Adonai for a soothing aroma. Its drink offering with it should be a quarter of a gallon[b] of wine. 14 You are not to eat bread, roasted grain, or fresh grain until this same day—until you have brought the offering of your God. It is a statute forever throughout your generations in all your dwellings.
15 “Then you are to count from the morrow after the Shabbat, from the day that you brought the omer[c] of the wave offering, seven complete Shabbatot. 16 Until the morrow after the seventh Shabbat you are to count fifty days,[d] and then present a new grain offering to Adonai. 17 You are to bring out of your houses two loaves of bread for a wave offering, made of two tenths of an ephah of fine flour. They are to be baked with hametz as firstfruits to Adonai. 18 You are to present, along with the bread, seven one-year-old lambs without blemish, one young bull, and two rams. They will become a burnt offering to Adonai, with their meal offering, and their drink offerings, an offering made by fire, a sweet aroma to Adonai. 19 Also you are to offer one male goat for a sin offering and a pair of year-old male lambs for a sacrifice of fellowship offerings. 20 The kohen is to wave them with the bread of the firstfruits as a wave offering before Adonai, with the two lambs. They shall be holy to Adonai for the kohen. 21 You are to make a proclamation on the same day that there is to be a holy convocation, and you should do no regular work. This is a statute forever in all your dwellings throughout your generations.