Paying Back Missed Fasts

- If a Muslim misses a day of fasting in Ramadan without a valid excuse, he must repent to Allah and seek His forgiveness; because the offense is great, and this is a grave abominable act. He must also in addition to repentance and seeking forgiveness, make up for the day(s) he did not fast after Ramadan. Here it is a necessity to make up for the day(s) immediately after Ramadan according to the most correct opinion of scholars, because he did not have a license to break his fast in the first place and he should have fasted the day(s) in its correct time.- If s/he broke his fast for a legitimate excuse, such as the reason of Hayd (menstruation) and nifaas (post-natal bleeding) for women, illness, travel or other excuses for which it is permissible to break the fast, then he must make up for the day(s), unless s/he is incapable of fasting. Moreover, it is not obligatory to fast these days immediately. Rather s/he has been given the chance to observe Fast over the entire following year, up until the next Ramadan, for �Aishah (may Allah be pleased with her) in a hadeeth said:
�I was not able to make up for the missed days of Ramadhan except in Sha�ban (the last month before the next Ramadhan).� A sub-narrator, Yahya, said, �She used to be busy serving the Prophet.�
[ Agreed upon.] However it is highly recommended and preferred to hasten to make up for the missed day(s), since by doing that one is discharging himself from that debt. And it is also safer for the person for something might unexpectedly happen, such as illness and the like, that would prevent him from fasting.
source: islamkingdomfasting