What is the ruling regarding someone who swore by Allah not to go to a specific person on a specific day, then doubted whether he had mentioned "the day" in his oath, and then went to him on another day; does he have to perform an expiation of fasting sixty days?

If a person intends a specific time with his oath but does not fulfill it, he does not break his oath, nor is he required to offer expiation. This is because the reference point for oaths is the intention of the swearer. Thus, if he intends with his oath something that the wording can bear, the oath is directed towards that intention.