The days between Pesach and Shavuos are considered days of mourning because during this time the 24,000 students of Rebbi Akiva died. According to the Midrash, the deaths occurred on 34 of these days. There are different customs as to which days to observe. The Rama 493-3 brings two customs. 1. From after Pesach until Lag B’omer. [On Chol Hamoed we do not observe mourning]. 2. From Rosh Chodesh Iyar until 3 days before Shavuos.
Originally, only some parts of Klal Yisrael had the custom not to take haircuts during these days. Since then, the custom has become universally accepted. However, being that it is a weaker custom, there are some leniencies. If Lag B’omer falls on a Friday, we can take haircuts on Thursday night instead of waiting until Friday morning. (see Mishna Berura 493-11). Also, of particular interest to us this year, when Lag B’omer falls out on Sunday, we can take haircuts already on Friday morning.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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