The idea of counting each day represents spiritual preparation and anticipation for the giving of the Torah, which was given by God on Mount Sinai at the beginning of the month of Sivan, around the same time as the holiday of Shavuot. The Sefer HaChinuch states that the Jewish people were only freed from Egypt at Passover in order to receive the Torah at Sinai, an event which is now celebrated on Shavuot, and to fulfill its laws. Thus the Counting of the Omer demonstrates how much a Jew desires to accept the Torah in his own life.
The origins of the omer count, enumerated in the Midrash Rabbah Parashas Emor, explains that when the Children of Israel left Egypt they were told by Moses that 49 days after the exodus, they would be given the Torah. The populace was so excited at the prospect of a spiritual liberation, following the physical emancipation from Egypt, they kept a count of the passing days that ended with the giving of the Torah at the foot of Mount Sinai. The Torah itself, in Leviticus 23:15-16, states that it is a commandment to count seven complete weeks from the day after Passover night ending with the festival of Shavuot on the fiftith day. Shavuot is the festival marking the giving of the Torah to the Jewish nation on the 6th of the Hebrew month of Sivan.
In keeping with the themes of spiritual growth and character development during this period, the Jewish sages compare the process of growth to the two types of grain offered at either pole of the counting period. In ancient times, barley was an animal food and wheat, a human food. At Passover, the Jews were raised out of the Egyptian exile although they had sunken almost to the point of no return. The Exodus was unearned, a gift from God, like the food of animals who are not expected to develop their spiritual potential. For the next forty-nine days, however, the Jewish people worked on themselves to be able to receive the Torah on their own merit. The receiving of the Torah required spiritual elevation and active cooperation. Thus the Shavuot offering is "people food"
Then he or she states the Omer-count in terms of both total days and weeks and days. For example, on the 23rd day the count would be stated thus: "Today is twenty-three days, which is three weeks and two days of (or "in" ) the Omer." The count is said in Hebrew.
According to the Halakha, a person may only recite the blessing while it is still night. If he or she remembers the count the next morning or afternoon, the count may still be made, but without a blessing. If one forgets to count a day altogether, he or she may continue to count succeeding days, but without a blessing.
"Omer-counters" are typically offered for sale during this time, and are displayed in synagogues for the benefit of worshippers who count the Omer with the congregation at the conclusion of evening services. Omer-counters range from decorative boxes with an interior scroll that shows each day's count through a small opening; to posters and magnets in which each day's count is recorded on a tear-off piece of paper; to calendars depicting all seven weeks and 49 days of the Omer (a small pointer is advanced from day to day); to pegboards that keep track of both the day and the week of the Omer. Reminders to count the Omer are also produced for hand-held computers and via SMS services for cell phones.
In Kabbalah, each of the seven weeks of the Omer-counting is associated with one of the seven lower sefirot (#4-10): Chesed, Gevurah, Tipheret, Netzach, Hod, Yesod, and Malchut. Each day of each week is also associated with one of these same seven sefirot, creating forty-nine permutations. The first day of the Omer is therefore associated with "chesed that is in chesed", the second day with "gevurah that is in chesed"; the first day of the second week is associated with "chesed that is in gevurah," the second day of the second week with "gevurah that is in gevurah," and so on. Symbolically, each of these 49 permutations represents an aspect of each person's character that can be improved or further developed. Rabbi Simon Jacobson explains these 49 levels in his classic book, The Spiritual Guide to Counting the Omer.
The forty-nine-day period of counting the Omer is also a conducive time to study the teaching of the Mishna in Pirkei Avoth 6:6, which enumerates the "48 ways" by which Torah is acquired. Rabbi Aharon Kotler explains that the study of each "way" can be done on each of the first forty-eight days of the Omer-counting; on the forty-ninth day, one should review all the "ways"
Lag Ba'omer, the thirty-third day of the Counting of the Omer, is considered to be the day in which the plague was lifted, (and/or: the day in which the rebellion saw a victory,) so on that day, all the rules of mourning are lifted (some Sephardim, however, continue the mourning period up until the 34th day of the Omer, which is considered by them to be the day of joy and celebration). Spanish and Portuguese Jews do not observe these customs. Some religious Jews will shave during the Omer period on Yom Ha'atzmaut, as they see this as a joyous occasion worthy of shaving. Although, after this they don't shave until the Omer is over.
In practice, different Jewish communities observe different periods of mourning. Some families listen to music during the week of Passover and then commence the period of mourning until Lag Ba'omer. Some Sephardic Jewish families begin the period of mourning from the first day of the Hebrew month of Iyar and continue for thirty-three days until the third of Sivan. The custom among Jerusalemites (minhag Yerushalmi) is to follow the mourning practices during the entire Counting of the Omer, save for the day of Lag Ba'omer and the last three days of the counting (sheloshet yemei hagbalah). The extent of mourning is also based heavily on family custom, and therefore Jews will mourn to different degrees.
Rabbi Yechiel Michel Epstein, author of Aruch Hashulchan, postulates that the mourning period also memorializes Jews who were murdered during the Crusades, pogroms and blood libels that occurred in Europe 1,000 years after the period of Rabbi Akiva
The Jewish calendar is largely agricultural, and the period of Omer falls between Passover and Shavuot. On Passover there is a shift from praying for rain to praying for dew and this begins the growth period for the fruit of the season. Shavuot is the day of the giving of the first fruits. The outcome of the season's crop and fruit was still vulnerable during this period. Over these seven weeks, daily reflection, work on one's middot (characteristics) and potential inner growth from this work on self was one way to pray for and invite the possibility of affecting one's external fate and potential - the growth of the crop and the fruit of that season.
According to the Zohar, Rabbi Shimon's house was filled with fire and light that entire day as he taught his students. At the end of the day, the fire subsided and Rabbi Shimon died On successive years, his students sought to recreate that experience of light and mystical revelation by kindling bonfires and studying the Zohar in the light of the flames.
Although the anniversary of the death of a tzaddik is usually a mournful day, the anniversary of Rabbi Shimon's death on Lag Ba'omer is a festive one. Bonfires are lit and people sing and dance by the flames. Weddings, parties, listening to music, picnics, and haircuts are commonplace.