2,096. A Fixed Time That Moves
Hilchos Issurei Biah 8:7
If a woman’s pattern was for her period to start on the fifteenth of the month but it moved to the sixteenth, marital intimacy is prohibited on both days. If it then moved to the seventeenth, the sixteenth becomes permitted and the seventeenth prohibited, with the fifteenth remaining prohibited. If it then moves to the eighteenth, the eighteenth becomes prohibited and all the other days permitted.
Hilchos Issurei Biah 8:8
If a woman’s pattern was for her period to start on the twentieth but it moved to the twenty-second, marital intimacy is prohibited on both days. If the twentieth arrives and her period doesn’t start until the twenty-second, both days remain prohibited. If the twentieth arrives again and her period begins, she has returned to her established pattern. The twenty-second becomes permitted because it was never established through three consecutive occurrences.