Date: 22 Adar II 5574
From: Rabbi Moshe Sofer
To: Unknown recipient
A son unsure whether his father died on the fourth or fifth of the month asks how to observe yahrzeit customs. The Chasam Sofer rules that if other children know the correct date, he observes only the first doubtful day.
Source: Shu"t Chasam Sofer OC §161