When a faster misses a day or days due to valid reasons mentioned previously, they should be redeemed after Ramadan. The redemption is mandatory, not immediately but delayed mandatory, known as "wajib 'at-Tarakhi." Although by this jurisprudent principle, there are eleven months to complete your Ramadan, it is preferred to make up these missed days as soon as possible, for Allah (SWT) says: "...Then strive together (as in a race) towards all that is good..." (Al-Qur`an, 2:148).
This verse indicates hastening to offer deeds is preferred for that is in the spirit of completing your Ramadan, and absolving the faster of the liability of unfasted days, rather than having it drag on for eleven months. After all, there is a possibility of death and missing the days altogether, as a result.
If there is reason to delay it, it is permitted to delay it no later than before the next Ramadan, or no later than the number of days he missed before the next Ramadan. If he missed five days, they should be redeemed not later than five days before the next Ramadan.
In an authenticated hadith attributed to 'Aishah (raa), she used to make up missed days in the month of Sha'aban, a month before Ramadan. If you miss more than one day you have the option to make them up consecutively or separately. Allah (SWT) states: " Fasting is for a fixed number of days. But if any of you is ill or on a journey, the missed number (of days) should be redeemed from days later...." (Al-Qur`an, 2:187) Thus, one who is ill or travels during Ramadan and does not fast, should fast days later to cover the ones missed, either in a group, all five days together, observed from Monday through Friday, or spread the five days over a whole month.
In a hadith related by Ibn 'Umar (raa), the Messenger of Allah said, referring to the redeeming of missed Ramadan fasts: "If he pleases, he redeems separately or consecutively." (Dar-Qutni)
No one should delay the making up of his missed days until next Ramadan unless there is a valid reason. In this event, one should fast during the present Ramadan, then make up the days missed in the previous Ramadan as soon as possible before the next Ramadan.