The Three days of Hagbalah

The three days of Hagbalah – the third, fourth, and fifth of Sivan – are the days when the Jews were forbidden to ascend the mountain before the giving of the Torah.

• Some have the custom to permit taking haircuts at this time. They consider that the mourning for the deaths of the students of Rabbi Akivah extends from Rosh Chodesh Iyar until the 3 Sivan. (Some consider the mourning from Pesach until Lag BaOmer). The Chabad custom (based on the Arizal), however, is not to take haircuts until Erev Shavuot.
• I have heard that, even according to Chabad custom, it is permissible to listen to music during these days.