Date: 8 Tishrei 5592
From: Rabbi Moshe Sofer
To: Rabbi Shmuel Zommer
During the 1831 cholera epidemic, some mourners could not observe aveilus due to overwhelming fear. The Chasam Sofer rules that if danger passed within thirty days, mourning must be observed retroactively; however, if a festival intervened before mourning was observed, the festival cancels the obligation entirely, since the penalty requiring make-up mourning applies to negligent non-observance but not to genuine compulsion (ones).
Source: Shu"t Chasam Sofer YD §342