Reference for Niddah 76:3

אימא

— Read: From Wednesday onwards.<span class="x" onmousemove="('comment',' But not on the nights preceding (and ritually belonging to) Sunday, Monday and Tuesday, since conception on any of these might result in a birth on a Sabbath which is the two hundred and seventy-third from a Sunday, the two hundred and seventy-second from a Monday and the two hundred and seventy-first from a Tuesday. ');">³</span>

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