What is the ruling for a woman who knows the month of her habitual menstruation but not the exact days?
Chapter on Menstruation
Al-Mughni
Book of Purification
Primary text
If a woman knows the month of her habitual menstruation, she must observe one period during each month of her habit. She reverts to her accustomed time frame, just as a woman with an established habit reverts to the established number of days. However, if her habitual month is less than twenty days, she is not to observe a period longer than the excess beyond thirteen or fifteen days. This is because observing a longer period would result in her purity (Tuhur) being less than the minimum required period, which is impermissible.
Supporting text
There are two scholarly views regarding the timing of this period observance within the month. One view mandates that she observes the period starting from the first day of the month, based on the Prophet's instruction to Humnah regarding the sequence of menstruation and purity days. The second view holds that she must determine the days through careful estimation and deliberation (Taharrī and Ijtihād), as the Prophet (peace be upon him) deferred the amount of time to estimation ('six or seven days'), suggesting the time frame should also be subject to estimation. If time estimation is equalized and nothing preponderates, reverting to the start of the month is necessary due to lack of evidence for alternatives.