Thursday night and Friday are the fifth day of the counting of the Omer, the measure of barley that was harvested in ancient times at the end of the 49-day counting period. The first picking of the crop was offered as a sacrifice in the Holy Temple. The blessing over the counting of the Omer is recited at night and is omitted during the following day if one forgot to count.
Singing and dancing are prohibited during most of the period in memory of 24,000 students of Rabbi Akiva who died in a plague during the same period. Rabbinic sages teach that the deaths were caused by senseless hatred.