Date: 15 Adar 5593
From: Rabbi Moshe Sofer
To: Rabbi Mordechai Schlank
A pious man who fasts every Monday and Thursday asks whether he may continue during Nissan. The Chasam Sofer rules he may not, upholding the Rema's strict prohibition against all fasting in Nissan.
Source: Shu"t Chasam Sofer OC §103