What is the ruling regarding a woman who vowed to fast for one day in a specific month, then forgot, then remembered and intended to fast, then was surprised by her menstrual period?

If you vowed to fast a day in a specific month, and you did not fast it until the month passed, you are obligated to make up for that day. You must also offer an expiation for breaking an oath (kaffarat yameen) as a precaution, due to a jurisprudential disagreement regarding its necessity. The expiation is to feed ten poor people, or clothe them, or free a slave. If you are unable to do so, then fasting three consecutive days is more cautious.