Question: Should a woman observe fast and abstain from all that breaks it in Ramadaan, if her menstruation ends and she becomes Taahir (ritually pure) after Fajr (Dawn) Prayer or after Thuhr (Noon) Prayer? Or must she not observe the fast in this circumstance, because the duration of fast is from the break of the dawn until sunset?
Answer: If menstruation ends and a woman becomes Taahir after the time when abstention from all that breaks Sawm enters, whether this occurs at the beginning of the day, in the middle of it, or at its end, then she has to abstain from all that breaks fast the rest of the day. This is according to the Saying of Allaah (Exalted be He): So whoever of you sights (the crescent on the first night of) the month (of Ramadaan i.e. is present at his home), he must observe Sawm (fasts) that month… read more here.