Sefirat HaOmer, also referred is as Sefira, is the period of 49 days between the second day of Jewish holiday of Passover and the holiday of Shavuot. The word "sefira", in Hebrew, literally translates to "counting", and the main ritual of this period is to count the days.
The origin of Sefirat HaOmer is found in the book of Leviticus, 23:15
usefartem lachem mi'machorat ha shabat mi'yom haviachem et-omer hatenufa sheva shabatot temimot tih'yeina
And you shall count for yourselves - from the day after the holiday, from the day on which the waved Omer Offering is brought, seven complete weeks.
Since according to the Jewish calendar, each day begins at the previous sunset, one should count each day on the evening that it begins. When counting the Omer, one should stand and recite the following blessing:
Baruch Ata Adon-y, Eloh-ynu Melech Ha-Olam, asher kiddeshanu b'mitzvotav, v'tzivanu al Sefirat HaOmer.
Blessed are You, Hashem, our G-d, King of the Universe, who sanctified us with His commandments, and commanded us regarding the Counting of the Omer.
After saying the blessing, one counts the current day. For example, to count the 10th day, one would say:
"Today is the 10th day, which is one week and three days, of the Omer"
If you forget to count at night, you may count on the next day, but without saying the blessing, and then resume counting as usual. If you forget at night and do not remember to count the next day, you should continue to count for the duration of Sefira, but do not say the blessing before counting.
Although this was not the orginal intent in the Bible, since the destruction of the Holy Temple, Sefirat HaOmer has become a period of mourning for the Jewish people. Although several tragedies occured during the period of Sefira, the primary reason for mourning is to commemorate the 12,000 students of Rabbi Akiva who died of a plague during the period between Passover and Shavuot.
As part of this mourning, many Jews do not cut their hair during this time period, get married, or listen to live music.
Since it is generally believed that the students stopped dying on the 33rd day of Sefira, this day is known as Lag BaOmer, and the restrictions of mourning are lifted at this time. In Israel and many Jewish communities, Lag BaOmer is a day of celebration traditionally marked by bonfires and picnics.