A vow means that a person obligates himself to do something which the Shariah does not ordain upon him. For example, he says: Upon me is a sacrifice or fasting such and such a day, if Allah cures this ill person of mine. So, it takes the form of stipulating a condition to Allah Almighty. In this Hadīth, the Tābi‘i Ziyād ibn Jubayr relates that a man came to ‘Abdullāh ibn ‘Umar (may Allah be pleased with him) and told him that he made a vow to fast a certain day, and this vowed day happened to coincide with the day of Eid al-Ad'ha, or the day of Eid al-Fitr. So, he asked him about what he should do: fast it or not? In response, Ibn ‘Umar (may Allah be pleased with him) said: "Allah Almighty enjoined the fulfillment of vows," which occurs in the verse that reads: {and fulfill their vows.} [Surat al-Hajj: 29] "and the Messenger of Allah (may Allah's peace and blessings be upon him) forbade fasting this day" i.e., fasting the two days of the two Eids. He forbade fasting these two days because they are days of festivity, play, joy, and eating and drinking. Also, Eid al-Fitr is the day of breaking the fast of Ramadan. As for the day of Eid al-Ad'ha, this is intended to enable the Muslim to eat from his sacrificial animal, if he has sacrificed. Ibn ‘Umar (may Allah be pleased with him) seemed to have refrained from giving a Fatwa on this issue, considering the conflict between the proofs he had. This was done by way of Tawarru‘ (caution) from Ibn ‘Umar (may Allah be pleased with him), and he was known for it. And it is said: Ibn ‘Umar (may Allah be pleased with him) clarified to him the general ruling and the specific one. So, he should do both things, namely, to not fast the day of Eid, in compliance with the prohibition by the Prophet (may Allah's peace and blessings be upon him), and to fast a day in place of it, given Allah's command to fulfill vows. The Hadīth contains the command to fulfill vows. It is prohibited to fast the two days of Eid al-Fitr and Eid al-Ad'ha..