What is the ruling regarding the days the one who made a vow did not fast, and what is the ruling regarding his breaking his fast on days when he is invited?

A vow is disliked and may cause its maker distress and sin, and failing to fulfill it is a grave matter. As for the question, the mentioned vow is an act of obedience that must be fulfilled, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." The person who made the vow must make up the days he missed, along with a single expiation for an oath for all the missed days. A lunch invitation is not considered an excuse for not fulfilling the vow.