Is it permissible to complete a missed fast by fasting unintentionally, and is it permissible to fast on the Day of Doubt?

Fasting to make up for Ramadan (definition) is like fasting Ramadan itself. So, whoever eats forgetfully while fasting should complete their fast. If you completed your day of fasting after eating forgetfully, then your obligation is fulfilled. Otherwise, you must make up that day. As for fasting on the Day of Doubt, it is not permissible, based on the hadith (definition) of Ammar ibn Yasir: "Whoever fasts on the Day of Doubt has disobeyed Abu al-Qasim (peace be upon him)." If fasting is obligatory, the demand for it does not drop except by ensuring its performance and the fulfillment of the obligation.