What is entailed for someone who vowed to fast the day after a certain action but was unable to perform it, and is expiation (kaffarah) due upon him after making up the fasting days?

Fulfilling a conditional vow becomes obligatory upon the occurrence of the condition. If you did not fast on the day immediately following the action to which the vow was tied due to an excuse like forgetfulness, then there is no make-up or expiation required of you. However, if the failure to fulfill the vow was without an excuse, the preponderant opinion is that you are only required to fast the missed vow without expiation. It should be noted that making a conditional vow is disliked and does not alter Allah's decree. The Prophet, peace and blessings be upon him, said: "Indeed, vows do not bring forth any good; they only extract from the miser."