Should one make up a fast for a specific day that was vowed and then forgotten?

The more cautious approach is to offer an expiation for an oath (kaffarat yameen) to avoid the scholarly disagreement regarding the obligation of expiation for one who breaks an oath forgetfully. It is obligatory to make up the day whose fasting was vowed but was forgotten, because a specified vow must be fulfilled if one is capable of doing so. It is reported in "Al-Taj wal-Iklil" that the well-known opinion is that one who breaks the fast of a specified vow forgetfully must make it up. They must also make it up if they break it due to travel or intentionally, but they do not need to make it up if they break it due to illness or menstruation.