Sunday is the 21st day of the counting of the Omer. The Omer is the measure of barley that was harvested in ancient times at the beginning of the 49-day counting period and brought to the Holy Temple as a sacrifice. The end of the counting period coincides with the evening before Shavuot, the festival of the first fruits and the handing down of the Torah on Har (Mount) Sinai. The counting of the Omer is recited at night, unless one forgot, in which case he or she may count during the day.

Most of the days between Passover and Shavuot are a period of partial mourning in memory of 24,000 students of Rabbi Akiva who died during the same period during the Bar Kochba uprising. One explanation teaches that some of Rabbi Akiva’s students joined the rebellion while others continued in their studies. The two camps behaved contemptuously towards one another, each claiming to be greater than the other, boasting that its behavior was more important and effective. They were stuck down before the enemy, according to this line of understanding, because of this groundless hatred.