Ziyad bin Jubair (may Allah have mercy on him) narrated that a man came to Sayyiduna Ibn Umar (may Allah be pleased with them) and said that a person made a vow to fast for one day, then thought that it would be a Monday, and by coincidence, that day turned out to be Eid. Sayyiduna Ibn Umar said that Allah Almighty has commanded the fulfillment of vows, and the Noble Prophet (peace and blessings of Allah be upon him) has forbidden fasting on that day (by Allah’s command).