In the period between Passover and Shavuot, we “count the omer.” An “omer” is a dry weight measure—slightly less than 4 pounds. When the Temple stood in Jerusalem, an omer of grain was brought each of the 49 days between the holidays. What we are counting, then, is not the omer itself, but the days during which the omer was brought.
This period is a time of mourning in the Jewish calendar. No weddings are performed, haircuts and shaving are forbidden, and some go so far as to not buy new clothes or listen to instrumental music.
There are different views of why and when exactly we mourn during sefirah (the period of counting), and therein lies a tale.
As regards the when, there are those who say the mourning starts from day one of sefirah, that is, from the second night of Passover, and extends to the holiday of lag b’omer—literally, the 33rd day of the omer.
There are others who say that the mourning starts from the first day of the Hebrew month of Iyar (which begins about a week after the end of Passover) and extends to the third day of Sivan (that is, three days before the holiday of Shavuot).
And there are those who combine the two aforesaid periods, and mourn from the second night of Passover to the third of Sivan.
And now, the “why”. Originally, the mourning period was in reaction to the death of 24 thousand students of Rabbi Akiva. There are several traditions to explain these deaths. One maintains that they suffered from a plague due to their failure to respect one another properly. The second links the deaths to the Bar Kochba rebellion against Rome, in which Rabbi Akiva was likely a participant. In both cases, the deaths stopped on the 33rd day of the omer—hence Lag b’omer, where “lag” is an abbreviation for 33. This remains the prominent Sefardi custom to this day.
And the matter would have probably stayed there, but then the Ashkenazi community went through the devastation of the Crusade of 1096 (otherwise known as the Rhineland massacre)—a series of attacks that began on the first of Iyar and lasted—you guessed it—for 33 days, until the third of Sivan.
The enormity of the devastation led the Ashkenazi authorities to shift the classic mourning period to match their own historical experience. This is a perfect example of how the Jewish tradition makes way for change by reflecting and adapting to current circumstances.
There are those in both the ashkenazic and sefardic communities who adhere to their own 33 day traditions. However, it has become common to combine both traditions, and extend the mourning period from Passover to the third of Sivan. This is a perfect example of another, and sometimes unfortunate, Jewish cultural trend—we’re great at addition, but mighty poor at subtraction.
One look at the siddur confirms this. Services used to be much shorter than they are today, but addition after addition has extended the time for davening (or for kibbitzing in shul, if that’s your thing…).
One final note—the conclusion of the mourning period on 3 Sivan corresponds beautifully with the preparations of our ancestors to receive the Torah at Mt. Sinai. It was starting on 3 Sivan that Moses began receiving instructions concerning the upcoming revelation. The days from 3 Sivan to Shavuot are known as the three days of hagbalah (literally: boundary setting), when Moses was supposed to set up barriers to prevent the people from approaching Hashem too closely.
The abrupt shift from the 33 days of mourning to the exultation of Torah at Sinai perfectly exemplifies the classic line from Psalm 30 “You changed my mourning into celebration.”
Torah does that, you know.