The ruling of fasting the days that one missed in Ramadan during the days in which fasting is prohibited
8-5-2016 | IslamWeb
Question:
My wife gave birth before the last Ramadan started. She is now fasting that she missed in Ramadan because of her delivery. She fasted during the days of Eid Al-Adh-ha, including the day of the Eid itself. What is the ruling of her fast?
Answer:
All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
It is prohibited for the Muslim to fast the days of Eed Al-Fitr and Eed Al-Adh-ha and the three days of tashreeq (the 11th, 12th, and 13th of Dhul-Hijjah) that come after the day of Eed Al-Adh-ha. Abu Sa’eed narrated that the Prophet, sallallaahu ‘alayhi wa sallam, prohibited fasting two days: the day of Al-Fitr and the day of Al-Adha. [Al-Bukhari and Muslim] In another narration by Ahmad and Al-Bukhari, the Prophet, sallallaahu ‘alayhi wa sallam, said: “No fasting is to take place in two days.” In a narration by Muslim, the Prophet, sallallaahu ‘alayhi wa sallam, said: “Fasting is not valid in two days.”
Ka’b Ibn Malik narrated that the Prophet, sallallaahu ‘alayhi wa sallam, sent him and Aws Ibn Al-Hadthaan to announce that none except believers would enter Paradise and that the days of Mina (days of Eed Al-Adh-ha) are for food and drink. [Ahmad and Muslim]
Sa’d Ibn Abu Waqqaas said, “The Prophet, sallallaahu ‘alayhi wa sallam, commanded me to announce: The days of Mina are days of eating and drinking; there should be no fasting in these days (days of tashreeq).” [Ahmad]
Anas 
said, “The Prophet, sallallaahu ‘alayhi wa sallam, prohibited the fasting of five days in the year: the Day of Al-Fitr, the Day of Sacrifice (Al-Adh-ha) and the three days of tashreeq.” [Ad-Daaraqutni]
‘Aa'ishah and Ibn ‘Umar said, “No one is allowed to fast the days of tashreeq, except for those (performing Hajj) who could not offer sacrificial animals.” [Al-Bukhari] They also said, “Fasting is required of the pilgrim who performs Tamattu` Hajj. If the pilgrim is not able to offer a sacrificial animal and is yet to expiate for this by fasting, then he may fast during the days of Mina.”
That being said, your wife’s fasting these days to make up for the days that she missed in Ramadan is not permissible and does not substitute for the days that she missed. However, she is not blameworthy for having done this if she did it out of ignorance about the prohibition of fasting these days.
Allaah Knows best