The Omer are forty-nine days between the holiday of Passover and the holiday of Shavuot. Also known as Sefirat HaOmer (Counting the Omer), these forty-nine days are counted aloud during evening services. First the service leader recites a special blessing that goes: "Blessed are You, Lord our God, Ruler of the Universe, who has commanded us to count the Omer." Then the congregation responds by saying: "Today is the third day [or fifth or thirtieth] day in the Omer." Shavuot is celebrated at the end of this period, on the fiftieth day after the second day of Passover.
An Ancient Custom
In Leviticus, the third book of the Torah, it says: "You shall count... from the day that you brought the omer as a wave offering" (23:15). "Omer" is a Hebrew word that means "sheaves of a harvested crop" and in ancient times Jews brought the omer to the Temple as an offering on the second day of Passover. The Torah tells us to count seven weeks from the bringing of the Omer until the evening of Shavuot, hence the custom of counting the Omer.
A Time of Semi-Mourning
Scholars are not sure why but historically the Omer has been a time of semi-mourning. The Talmud mentions a plague that is thought to have killed 24,000 of Rabbi Akiva's students during one Omer, and some think this is the reason the Omer are not joyous. Others wonder if this "plague" may have been code for another disaster: Rabbi Akiva's support of Simon Bar-Kokhba's failed rebellion against the Romans. It is possible that these 24,000 students died fighting in battle.
Because of the somber tone of the Omer, traditional Jews do not get haircuts or celebrate weddings during this period. The one exception to this rule is Lag B'Omer