Date: Chanukah 5597
From: Rabbi Moshe Sofer
To: Rabbi Shaul Landau
During a cholera epidemic in Krakow, clouds obscured the moon until the 16th of Kislev, past the usual deadline for Birkas HaLevanah. The Chasam Sofer validates reciting the blessing on the 16th night through multiple arguments.
Source: Shu"t Chasam Sofer OC §102