Reference for Niddah 127:28

הכא במאי עסקינן כגון דרגילה למחזי ביום עשרים ושינתה ליום עשרים ואחד

As it might have been presumed that since she was accustomed to observe her discharge on a settled date and this was changed, the change is effective<span class="x" onmousemove="('comment',' Sc. the first date is no longer regarded as a settled period. ');">²⁶</span>

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