Haircutting during the Three Days of Preparation, before Erev Shavuot, was displeasing to my father.

Whoever ponders the approach of Tosafot in his comment beginning "Torah etc..." (Shabbat 89a) will understand that the festival of Shavuot is an auspicious time Above. On that day G-d confounds the "supernal accuser" of Israel, similar to His confounding the accuser during Shofar-sounding on Rosh Hashana and on the holy day of the Fast of Yom Kippur.