A:
Anyone who observed Sawm on Thursday should make up for the day missed at the beginning of Ramadan, as they did not observe it intending it to be part of the month of Ramadan. Moreover, they observed Sawm on this day believing it was the Day of Doubt (so called because there is doubt concerning it - is it the last day of Sha‘ban or the first day of Ramadan).
It is not permissible to observe Sawm on the Day of Doubt. Fasting the Day of Doubt is not sufficient if it is later discovered that this day was part of Ramadan. (Part No. 10; Page No. 119) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.