From: Rabbi Moshe Sofer
To: Rabbi Shlomo
A man who fasts annually on his parent's yahrzeit has forgotten whether the correct date is the 25th or 26th of Sivan. The Chasam Sofer rules he should fast on the earlier date (the 25th), applying the principle of zerizin makdimin.
Source: Shu"t Chasam Sofer OC §162