If menstruation is ongoing at dawn, a valid Ramadan fast cannot be commenced for that day. The day is later made up after attaining purity.
This differs in timing from menstruation beginning mid-day, though both lead to make-up. Fasting intention alone does not override the impediment.
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Kitab al-Sawm: Unconsciousness, Recovery, Menstruation and Abstention
If menstruation is ongoing at dawn, a valid Ramadan fast cannot be commenced for that day. The day is later made up after attaining purity.
This differs in timing from menstruation beginning mid-day, though both lead to make-up. Fasting intention alone does not override the impediment.