Due to sickness or other reasons, we might have missed some days of fasting in Ramadan. When is the best time to make up those fasts, and when do we have to pay fidyah or kaffrah, the amounts paid in place of making up fasts?
It is important to make up your fasts as soon as possible, unless you have a valid reason for not fasting, and you are unable to do so, then you pay fidyah...
Shaykh Yasir Qadhi explains further
If you have a legitimate excuse that caused you to miss a few days of Ramadan, you should make up the fast as soon as possible, preferably before the next Ramadan. As for paying Fidyah or Kaffarah, if you are not fasting for a legitimate reason and you're not going to make up the fast, so for example you're elderly or you're sick, you should pay the Fidyah or the Kaffarah on that very day.
Or you can prepay or you can pay at the end of the month all at once. But it should be done during that month as soon as possible. And preferably, ideally every day you pay one person the way it should be done. But it is allowed to pay a lump sum. So you take all of the 30 days and then you give to the poor at the beginning or at the end of the month.