The command in Leviticus
The practice of counting the Omer is rooted in a passage in Leviticus 23. After describing Passover, the text tells the people to bring an omer, a measure of the first grain of the barley harvest, as an offering. It then instructs them to count seven full weeks “from the day after the sabbath”, and to bring a new grain offering on the fiftieth day. The word omer refers both to the sheaf and to the quantity of grain, and it has given the whole period its name.
Everything in the practice follows from that counting: seven weeks make 49 days, and the day after that, the fiftieth, is the festival of Shavuot, the “Feast of Weeks”. The Greek name for the fiftieth day, Pentecost, comes from the same arithmetic. The festival ties the barley harvest of spring to the wheat harvest of early summer, and the count joins the two.
Which sabbath?
The phrase “the day after the sabbath” has been read in more than one way. In the rabbinic tradition, which became standard in Judaism, “sabbath” here means the first day of Passover, a festival day, so the count begins on the second day of Passover, the sixteenth of Nisan. Other groups in the Second Temple period are reported to have understood the phrase as the weekly Sabbath, which would move the start of the count and the date of Shavuot. The details of those disputes are known mainly through later sources, so it is safest to say simply that the question was debated and that the rabbinic reading prevailed.
How the count is performed
In the traditional practice, the count is made each evening after nightfall, starting on the second night of Passover and continuing for 49 nights. A blessing is said, followed by a formula stating the day in two ways: the total number of days, and, from day seven onward, the completed weeks and extra days. On day 22, for example, the count is 22 days, which is three weeks and one day. Forms of the wording vary by community, and the practice is often written down in prayer books so that no one loses track. Tradition holds that if a day is missed entirely, the later counting may need to proceed without the blessing, a point treated differently in different communities.
The structure of seven times seven has its own appeal. Seven is the number of the week and of the sabbatical cycle described in the Bible, and forty-nine is its square. It builds on a pattern that also underlies the seven-day week, and on the wider place of seven in antiquity; the Omer count brings those threads together in a stretch of time that is entirely counted.
A period of semi-mourning
In many communities, the Omer period is observed as a time of partial mourning. Customs often include avoiding weddings, public celebrations with music, and haircuts, though the specific dates and the degree of restriction vary. The Talmud connects the mourning to a plague said to have killed many students of Rabbi Akiva in the period between Passover and Shavuot. Medieval authorities adopted and adapted the tradition in different ways, and a number of different dates for when the restrictions begin and end are followed today. As with many mourning periods in Jewish tradition, the number of days is a matter of custom and local practice, not a single universal rule.
Lag BaOmer
The thirty-third day of the count is marked as Lag BaOmer; “Lag” is the Hebrew letter-number for 33, written with lamed (30) and gimel (3). In many communities the mourning restrictions are lifted or relaxed on this day. It is associated in later tradition with the sage Shimon bar Yochai, and it is celebrated with bonfires, outings and, in some places, weddings and haircuts. The reasons given for the day vary, and several explanations circulate. Historians of the holiday note that some of them developed centuries after the events they refer to, so they are best read as traditions rather than established facts.
Mystical readings
Kabbalistic writers gave the count another layer. Each of the seven weeks is associated with one of the lower qualities in the Kabbalistic sefirot, and each day of the week with one of those same seven again, so that every day of the 49 has its own pairing of qualities. The practice is used as a framework for reflection and self-examination in some communities. It is a late devotional overlay on the older agricultural count and should be seen as a belief about spiritual preparation, not an effect that can be measured.
The Omer and the Hebrew calendar
Counting by days between two festivals also depends on the calendar being stable. The Hebrew calendar is lunisolar: months follow the moon, and leap months are added in a nineteen-year pattern so that Passover stays in the spring. The same cycle of nineteen years is known more widely as the Metonic cycle. Because Passover is fixed to the spring season in this way, the harvest connection of the Omer is preserved even though the count is made in lunar months. In this respect the count is one of the clearest examples of how calendar systems turn farming seasons into religious time.
After the harvest, a new meaning
In modern Jewish practice, Shavuot is associated with the giving of the Torah at Sinai, a link made in rabbinic literature rather than in Leviticus itself. On this understanding, the 49 days become a period of anticipation leading from the liberation commemorated at Passover to the revelation commemorated at Shavuot. The agricultural and the historical readings coexist, and different communities emphasize one or the other.