From: Rabbi Moshe Sofer
To: Unknown recipient
May a community establish a commemorative Yom Tov for a miracle that happened to them, and does it carry binding halachic force? The Chasam Sofer rules such observances are obligatory, siding with the Magen Avraham against the Pri Chadash.
Source: Shu"t Chasam Sofer OC §191