Making up missed days of Ramadan is an obligation that need not be fulfilled immediately because the time for fulfilling is very wide and one may perform it at any time. This is also the case with the fast of expiation. It has been authentically reported that ‘Aishah would make up her missed days during the month of Sha’ban (the month preceding Ramadan), and that she did not perform them immediately even if she had the ability to do so.
Observing the fast of Ramadan and making up the days are the same with respect to the fact that if one day of Ramadan is missed, then only one day needs to be made up. There is no additional penalty. They differ about the fact that when a person makes up the missed days he need not do so on consecutive days. This is because Allah says: “For him who is sick or on a journey, [the same] number of other days” – that is, whoever is sick or travelling and breaks the fast must fast the same number of days that he missed, consecutively or un-consecutively.
Allah has ordered the fast in a general manner without any restricting clauses.
As for making up the missed days of Ramadan, ad-Daraqutni recorded from Ibn ‘Umar that the Prophet said: “If you wish, make them on non-consecutive days and if you wish on consecutive days.”
If one delays performing the missed days of fasting until the next Ramadan comes, he is to fast the present Ramadan and then make up the days from the previous Ramadan. There is no ransom payment to be made, regardless of whether the person delayed the fasting due to some acceptable excuse or not. This is the opinion of the Hanafiyyah and al-Hassan al-Basri. Malik, ash-Shaf’i, Ahmad, and Ishaq agree that there is no ransom payment if the fasting was delayed due to some excuse, but they differ when the fasting was delayed without any acceptable excuse. In such a case, according to them, the person should fast the present Ramadan and then make up the days he missed from the previous Ramadan along with a ransom payment of a mudd of food given in charity each day. It should be noted that they have no acceptable evidence for that opinion. Apparently, the correct opinion is that of the Hanafiyyah, as there is no lawmaking without an authentic legal text to support it (that is, a Qur’anic verse or hadith).