From: Rabbi Moshe Sofer
To: Unknown recipient
When a mourner for a parent reaches the ge'arah threshold coinciding with the onset of the Three Weeks, the Chasam Sofer rules that although the Bach permits scissors in overlapping mourning situations, one should not take a full festive haircut. Instead, on the 18th of Tammuz, the mourner should take only a minimal trim sufficient to remove the unkempt appearance warranting rebuke. He further argues that for Ashkenazi Jews who regularly shave their beards, thirty days of growth reaches the ge'arah threshold faster than twelve months of uncut head hair, collapsing the practical distinction between sheloshim and ge'arah for Ashkenazim.
Source: Shu"t Chasam Sofer YD §347