Fasting: Not Knowing Whether it is the Last Day of Ramadan or the First of Shawwal
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1705
If a person does not know whether it is the last day of the month of Ramadan or the first of Shawwāl, he must fast on that day. However, if during the day he finds out that it is the first of Shawwāl, he must break his fast.