The sad mood of these weeks is ascribed to the death of thousands of Rabbi Akiva’s students from an epidemic. Because Rabbi Akiva’s support was crucial to the Bar Kochba revolt against the Romans, the loss of his adherents was an especial tragedy for the Jewish national cause.
Other calamities in Jewish history also occurred during this time of the year. It is believed that the epidemic in Roman times abated on the 33rd day, which accounts for the happier mood of Lag Ba’Omer.
Yet there still remains a problem in that in many traditions the sadness resumes the following day.
A novel interpretation is given by some who note that the pilgrimages to the Temple for Pesach, Shavu’ot and Sukkot were particularly difficult when people went for Pesach, returned home for a few days and then set out again for Shavu’ot.
It is possible that despite the strain of travelling they were keyed up on 18 Iyyar when their next journey to Jerusalem began.