Date: 15 Elul 5559
From: Rabbi Moshe Sofer
To: Unknown recipient
A woman with a fifteen-year pattern of seeing blood between days 26–28 of her cycle completed her seven clean days on day 27 from her most recent flow. The Chasam Sofer rules that if no single day within the range was established as a fixed veset through three consecutive identical occurrences, she may immerse and resume relations before the 19th of Elul; if day 28 was ever fixed and never uprooted, she must separate on the night of the 16th of Elul; and in cases of doubt, he rules stringently, with a limited leniency for women who can confirm their sightings always began in the daytime.
Source: Shu"t Chasam Sofer YD §166